// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * Budget Fair Queueing (BFQ) I/O scheduler.
 *
 * Based on ideas and code from CFQ:
 * Copyright (C) 2003 Jens Axboe <axboe@kernel.dk>
 *
 * Copyright (C) 2008 Fabio Checconi <fabio@gandalf.sssup.it>
 *                    Paolo Valente <paolo.valente@unimore.it>
 *
 * Copyright (C) 2010 Paolo Valente <paolo.valente@unimore.it>
 *                    Arianna Avanzini <avanzini@google.com>
 *
 * Copyright (C) 2017 Paolo Valente <paolo.valente@linaro.org>
 *
 * BFQ is a proportional-share I/O scheduler, with some extra
 * low-latency capabilities. BFQ also supports full hierarchical
 * scheduling through cgroups. Next paragraphs provide an introduction
 * on BFQ inner workings. Details on BFQ benefits, usage and
 * limitations can be found in Documentation/block/bfq-iosched.rst.
 *
 * BFQ is a proportional-share storage-I/O scheduling algorithm based
 * on the slice-by-slice service scheme of CFQ. But BFQ assigns
 * budgets, measured in number of sectors, to processes instead of
 * time slices. The device is not granted to the in-service process
 * for a given time slice, but until it has exhausted its assigned
 * budget. This change from the time to the service domain enables BFQ
 * to distribute the device throughput among processes as desired,
 * without any distortion due to throughput fluctuations, or to device
 * internal queueing. BFQ uses an ad hoc internal scheduler, called
 * B-WF2Q+, to schedule processes according to their budgets. More
 * precisely, BFQ schedules queues associated with processes. Each
 * process/queue is assigned a user-configurable weight, and B-WF2Q+
 * guarantees that each queue receives a fraction of the throughput
 * proportional to its weight. Thanks to the accurate policy of
 * B-WF2Q+, BFQ can afford to assign high budgets to I/O-bound
 * processes issuing sequential requests (to boost the throughput),
 * and yet guarantee a low latency to interactive and soft real-time
 * applications.
 *
 * In particular, to provide these low-latency guarantees, BFQ
 * explicitly privileges the I/O of two classes of time-sensitive
 * applications: interactive and soft real-time. In more detail, BFQ
 * behaves this way if the low_latency parameter is set (default
 * configuration). This feature enables BFQ to provide applications in
 * these classes with a very low latency.
 *
 * To implement this feature, BFQ constantly tries to detect whether
 * the I/O requests in a bfq_queue come from an interactive or a soft
 * real-time application. For brevity, in these cases, the queue is
 * said to be interactive or soft real-time. In both cases, BFQ
 * privileges the service of the queue, over that of non-interactive
 * and non-soft-real-time queues. This privileging is performed,
 * mainly, by raising the weight of the queue. So, for brevity, we
 * call just weight-raising periods the time periods during which a
 * queue is privileged, because deemed interactive or soft real-time.
 *
 * The detection of soft real-time queues/applications is described in
 * detail in the comments on the function
 * bfq_bfqq_softrt_next_start. On the other hand, the detection of an
 * interactive queue works as follows: a queue is deemed interactive
 * if it is constantly non empty only for a limited time interval,
 * after which it does become empty. The queue may be deemed
 * interactive again (for a limited time), if it restarts being
 * constantly non empty, provided that this happens only after the
 * queue has remained empty for a given minimum idle time.
 *
 * By default, BFQ computes automatically the above maximum time
 * interval, i.e., the time interval after which a constantly
 * non-empty queue stops being deemed interactive. Since a queue is
 * weight-raised while it is deemed interactive, this maximum time
 * interval happens to coincide with the (maximum) duration of the
 * weight-raising for interactive queues.
 *
 * Finally, BFQ also features additional heuristics for
 * preserving both a low latency and a high throughput on NCQ-capable,
 * rotational or flash-based devices, and to get the job done quickly
 * for applications consisting in many I/O-bound processes.
 *
 * NOTE: if the main or only goal, with a given device, is to achieve
 * the maximum-possible throughput at all times, then do switch off
 * all low-latency heuristics for that device, by setting low_latency
 * to 0.
 *
 * BFQ is described in [1], where also a reference to the initial,
 * more theoretical paper on BFQ can be found. The interested reader
 * can find in the latter paper full details on the main algorithm, as
 * well as formulas of the guarantees and formal proofs of all the
 * properties.  With respect to the version of BFQ presented in these
 * papers, this implementation adds a few more heuristics, such as the
 * ones that guarantee a low latency to interactive and soft real-time
 * applications, and a hierarchical extension based on H-WF2Q+.
 *
 * B-WF2Q+ is based on WF2Q+, which is described in [2], together with
 * H-WF2Q+, while the augmented tree used here to implement B-WF2Q+
 * with O(log N) complexity derives from the one introduced with EEVDF
 * in [3].
 *
 * [1] P. Valente, A. Avanzini, "Evolution of the BFQ Storage I/O
 *     Scheduler", Proceedings of the First Workshop on Mobile System
 *     Technologies (MST-2015), May 2015.
 *     http://algogroup.unimore.it/people/paolo/disk_sched/mst-2015.pdf
 *
 * [2] Jon C.R. Bennett and H. Zhang, "Hierarchical Packet Fair Queueing
 *     Algorithms", IEEE/ACM Transactions on Networking, 5(5):675-689,
 *     Oct 1997.
 *
 * http://www.cs.cmu.edu/~hzhang/papers/TON-97-Oct.ps.gz
 *
 * [3] I. Stoica and H. Abdel-Wahab, "Earliest Eligible Virtual Deadline
 *     First: A Flexible and Accurate Mechanism for Proportional Share
 *     Resource Allocation", technical report.
 *
 * http://www.cs.berkeley.edu/~istoica/papers/eevdf-tr-95.pdf
 */
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/blkdev.h>
#include <linux/cgroup.h>
#include <linux/elevator.h>
#include <linux/ktime.h>
#include <linux/rbtree.h>
#include <linux/ioprio.h>
#include <linux/sbitmap.h>
#include <linux/delay.h>
#include <linux/backing-dev.h>

#include "blk.h"
#include "blk-mq.h"
#include "blk-mq-tag.h"
#include "blk-mq-sched.h"
#include "bfq-iosched.h"
#include "blk-wbt.h"

#define BFQ_BFQQ_FNS(name)                                              \
void bfq_mark_bfqq_##name(struct bfq_queue *bfqq)                       \
{                                                                       \
        __set_bit(BFQQF_##name, &(bfqq)->flags);                        \
}                                                                       \
void bfq_clear_bfqq_##name(struct bfq_queue *bfqq)                      \
{                                                                       \
        __clear_bit(BFQQF_##name, &(bfqq)->flags);              \
}                                                                       \
int bfq_bfqq_##name(const struct bfq_queue *bfqq)                       \
{                                                                       \
        return test_bit(BFQQF_##name, &(bfqq)->flags);          \
}

BFQ_BFQQ_FNS(just_created);
BFQ_BFQQ_FNS(busy);
BFQ_BFQQ_FNS(wait_request);
BFQ_BFQQ_FNS(non_blocking_wait_rq);
BFQ_BFQQ_FNS(fifo_expire);
BFQ_BFQQ_FNS(has_short_ttime);
BFQ_BFQQ_FNS(sync);
BFQ_BFQQ_FNS(IO_bound);
BFQ_BFQQ_FNS(in_large_burst);
BFQ_BFQQ_FNS(coop);
BFQ_BFQQ_FNS(split_coop);
BFQ_BFQQ_FNS(softrt_update);
BFQ_BFQQ_FNS(has_waker);
#undef BFQ_BFQQ_FNS                                             \

/* Expiration time of sync (0) and async (1) requests, in ns. */
static const u64 bfq_fifo_expire[2] = { NSEC_PER_SEC / 4, NSEC_PER_SEC / 8 };

/* Maximum backwards seek (magic number lifted from CFQ), in KiB. */
static const int bfq_back_max = 16 * 1024;

/* Penalty of a backwards seek, in number of sectors. */
static const int bfq_back_penalty = 2;

/* Idling period duration, in ns. */
static u64 bfq_slice_idle = NSEC_PER_SEC / 125;

/* Minimum number of assigned budgets for which stats are safe to compute. */
static const int bfq_stats_min_budgets = 194;

/* Default maximum budget values, in sectors and number of requests. */
static const int bfq_default_max_budget = 16 * 1024;

/*
 * When a sync request is dispatched, the queue that contains that
 * request, and all the ancestor entities of that queue, are charged
 * with the number of sectors of the request. In contrast, if the
 * request is async, then the queue and its ancestor entities are
 * charged with the number of sectors of the request, multiplied by
 * the factor below. This throttles the bandwidth for async I/O,
 * w.r.t. to sync I/O, and it is done to counter the tendency of async
 * writes to steal I/O throughput to reads.
 *
 * The current value of this parameter is the result of a tuning with
 * several hardware and software configurations. We tried to find the
 * lowest value for which writes do not cause noticeable problems to
 * reads. In fact, the lower this parameter, the stabler I/O control,
 * in the following respect.  The lower this parameter is, the less
 * the bandwidth enjoyed by a group decreases
 * - when the group does writes, w.r.t. to when it does reads;
 * - when other groups do reads, w.r.t. to when they do writes.
 */
static const int bfq_async_charge_factor = 3;

/* Default timeout values, in jiffies, approximating CFQ defaults. */
const int bfq_timeout = HZ / 8;

/*
 * Time limit for merging (see comments in bfq_setup_cooperator). Set
 * to the slowest value that, in our tests, proved to be effective in
 * removing false positives, while not causing true positives to miss
 * queue merging.
 *
 * As can be deduced from the low time limit below, queue merging, if
 * successful, happens at the very beginning of the I/O of the involved
 * cooperating processes, as a consequence of the arrival of the very
 * first requests from each cooperator.  After that, there is very
 * little chance to find cooperators.
 */
static const unsigned long bfq_merge_time_limit = HZ/10;

static struct kmem_cache *bfq_pool;

/* Below this threshold (in ns), we consider thinktime immediate. */
#define BFQ_MIN_TT              (2 * NSEC_PER_MSEC)

/* hw_tag detection: parallel requests threshold and min samples needed. */
#define BFQ_HW_QUEUE_THRESHOLD  3
#define BFQ_HW_QUEUE_SAMPLES    32

#define BFQQ_SEEK_THR           (sector_t)(8 * 100)
#define BFQQ_SECT_THR_NONROT    (sector_t)(2 * 32)
#define BFQ_RQ_SEEKY(bfqd, last_pos, rq) \
        (get_sdist(last_pos, rq) >                      \
         BFQQ_SEEK_THR &&                               \
         (!blk_queue_nonrot(bfqd->queue) ||             \
          blk_rq_sectors(rq) < BFQQ_SECT_THR_NONROT))
#define BFQQ_CLOSE_THR          (sector_t)(8 * 1024)
#define BFQQ_SEEKY(bfqq)        (hweight32(bfqq->seek_history) > 19)
/*
 * Sync random I/O is likely to be confused with soft real-time I/O,
 * because it is characterized by limited throughput and apparently
 * isochronous arrival pattern. To avoid false positives, queues
 * containing only random (seeky) I/O are prevented from being tagged
 * as soft real-time.
 */
#define BFQQ_TOTALLY_SEEKY(bfqq)        (bfqq->seek_history == -1)

/* Min number of samples required to perform peak-rate update */
#define BFQ_RATE_MIN_SAMPLES    32
/* Min observation time interval required to perform a peak-rate update (ns) */
#define BFQ_RATE_MIN_INTERVAL   (300*NSEC_PER_MSEC)
/* Target observation time interval for a peak-rate update (ns) */
#define BFQ_RATE_REF_INTERVAL   NSEC_PER_SEC

/*
 * Shift used for peak-rate fixed precision calculations.
 * With
 * - the current shift: 16 positions
 * - the current type used to store rate: u32
 * - the current unit of measure for rate: [sectors/usec], or, more precisely,
 *   [(sectors/usec) / 2^BFQ_RATE_SHIFT] to take into account the shift,
 * the range of rates that can be stored is
 * [1 / 2^BFQ_RATE_SHIFT, 2^(32 - BFQ_RATE_SHIFT)] sectors/usec =
 * [1 / 2^16, 2^16] sectors/usec = [15e-6, 65536] sectors/usec =
 * [15, 65G] sectors/sec
 * Which, assuming a sector size of 512B, corresponds to a range of
 * [7.5K, 33T] B/sec
 */
#define BFQ_RATE_SHIFT          16

/*
 * When configured for computing the duration of the weight-raising
 * for interactive queues automatically (see the comments at the
 * beginning of this file), BFQ does it using the following formula:
 * duration = (ref_rate / r) * ref_wr_duration,
 * where r is the peak rate of the device, and ref_rate and
 * ref_wr_duration are two reference parameters.  In particular,
 * ref_rate is the peak rate of the reference storage device (see
 * below), and ref_wr_duration is about the maximum time needed, with
 * BFQ and while reading two files in parallel, to load typical large
 * applications on the reference device (see the comments on
 * max_service_from_wr below, for more details on how ref_wr_duration
 * is obtained).  In practice, the slower/faster the device at hand
 * is, the more/less it takes to load applications with respect to the
 * reference device.  Accordingly, the longer/shorter BFQ grants
 * weight raising to interactive applications.
 *
 * BFQ uses two different reference pairs (ref_rate, ref_wr_duration),
 * depending on whether the device is rotational or non-rotational.
 *
 * In the following definitions, ref_rate[0] and ref_wr_duration[0]
 * are the reference values for a rotational device, whereas
 * ref_rate[1] and ref_wr_duration[1] are the reference values for a
 * non-rotational device. The reference rates are not the actual peak
 * rates of the devices used as a reference, but slightly lower
 * values. The reason for using slightly lower values is that the
 * peak-rate estimator tends to yield slightly lower values than the
 * actual peak rate (it can yield the actual peak rate only if there
 * is only one process doing I/O, and the process does sequential
 * I/O).
 *
 * The reference peak rates are measured in sectors/usec, left-shifted
 * by BFQ_RATE_SHIFT.
 */
static int ref_rate[2] = {14000, 33000};
/*
 * To improve readability, a conversion function is used to initialize
 * the following array, which entails that the array can be
 * initialized only in a function.
 */
static int ref_wr_duration[2];

/*
 * BFQ uses the above-detailed, time-based weight-raising mechanism to
 * privilege interactive tasks. This mechanism is vulnerable to the
 * following false positives: I/O-bound applications that will go on
 * doing I/O for much longer than the duration of weight
 * raising. These applications have basically no benefit from being
 * weight-raised at the beginning of their I/O. On the opposite end,
 * while being weight-raised, these applications
 * a) unjustly steal throughput to applications that may actually need
 * low latency;
 * b) make BFQ uselessly perform device idling; device idling results
 * in loss of device throughput with most flash-based storage, and may
 * increase latencies when used purposelessly.
 *
 * BFQ tries to reduce these problems, by adopting the following
 * countermeasure. To introduce this countermeasure, we need first to
 * finish explaining how the duration of weight-raising for
 * interactive tasks is computed.
 *
 * For a bfq_queue deemed as interactive, the duration of weight
 * raising is dynamically adjusted, as a function of the estimated
 * peak rate of the device, so as to be equal to the time needed to
 * execute the 'largest' interactive task we benchmarked so far. By
 * largest task, we mean the task for which each involved process has
 * to do more I/O than for any of the other tasks we benchmarked. This
 * reference interactive task is the start-up of LibreOffice Writer,
 * and in this task each process/bfq_queue needs to have at most ~110K
 * sectors transferred.
 *
 * This last piece of information enables BFQ to reduce the actual
 * duration of weight-raising for at least one class of I/O-bound
 * applications: those doing sequential or quasi-sequential I/O. An
 * example is file copy. In fact, once started, the main I/O-bound
 * processes of these applications usually consume the above 110K
 * sectors in much less time than the processes of an application that
 * is starting, because these I/O-bound processes will greedily devote
 * almost all their CPU cycles only to their target,
 * throughput-friendly I/O operations. This is even more true if BFQ
 * happens to be underestimating the device peak rate, and thus
 * overestimating the duration of weight raising. But, according to
 * our measurements, once transferred 110K sectors, these processes
 * have no right to be weight-raised any longer.
 *
 * Basing on the last consideration, BFQ ends weight-raising for a
 * bfq_queue if the latter happens to have received an amount of
 * service at least equal to the following constant. The constant is
 * set to slightly more than 110K, to have a minimum safety margin.
 *
 * This early ending of weight-raising reduces the amount of time
 * during which interactive false positives cause the two problems
 * described at the beginning of these comments.
 */
static const unsigned long max_service_from_wr = 120000;

#define RQ_BIC(rq)              icq_to_bic((rq)->elv.priv[0])
#define RQ_BFQQ(rq)             ((rq)->elv.priv[1])

struct bfq_queue *bic_to_bfqq(struct bfq_io_cq *bic, bool is_sync)
{
        return bic->bfqq[is_sync];
}

void bic_set_bfqq(struct bfq_io_cq *bic, struct bfq_queue *bfqq, bool is_sync)
{
        bic->bfqq[is_sync] = bfqq;
}

struct bfq_data *bic_to_bfqd(struct bfq_io_cq *bic)
{
        return bic->icq.q->elevator->elevator_data;
}

/**
 * icq_to_bic - convert iocontext queue structure to bfq_io_cq.
 * @icq: the iocontext queue.
 */
static struct bfq_io_cq *icq_to_bic(struct io_cq *icq)
{
        /* bic->icq is the first member, %NULL will convert to %NULL */
        return container_of(icq, struct bfq_io_cq, icq);
}

/**
 * bfq_bic_lookup - search into @ioc a bic associated to @bfqd.
 * @bfqd: the lookup key.
 * @ioc: the io_context of the process doing I/O.
 * @q: the request queue.
 */
static struct bfq_io_cq *bfq_bic_lookup(struct bfq_data *bfqd,
                                        struct io_context *ioc,
                                        struct request_queue *q)
{
        if (ioc) {
                unsigned long flags;
                struct bfq_io_cq *icq;

                spin_lock_irqsave(&q->queue_lock, flags);
                icq = icq_to_bic(ioc_lookup_icq(ioc, q));
                spin_unlock_irqrestore(&q->queue_lock, flags);

                return icq;
        }

        return NULL;
}

/*
 * Scheduler run of queue, if there are requests pending and no one in the
 * driver that will restart queueing.
 */
void bfq_schedule_dispatch(struct bfq_data *bfqd)
{
        if (bfqd->queued != 0) {
                bfq_log(bfqd, "schedule dispatch");
                blk_mq_run_hw_queues(bfqd->queue, true);
        }
}

#define bfq_class_idle(bfqq)    ((bfqq)->ioprio_class == IOPRIO_CLASS_IDLE)

#define bfq_sample_valid(samples)       ((samples) > 80)

/*
 * Lifted from AS - choose which of rq1 and rq2 that is best served now.
 * We choose the request that is closer to the head right now.  Distance
 * behind the head is penalized and only allowed to a certain extent.
 */
static struct request *bfq_choose_req(struct bfq_data *bfqd,
                                      struct request *rq1,
                                      struct request *rq2,
                                      sector_t last)
{
        sector_t s1, s2, d1 = 0, d2 = 0;
        unsigned long back_max;
#define BFQ_RQ1_WRAP    0x01 /* request 1 wraps */
#define BFQ_RQ2_WRAP    0x02 /* request 2 wraps */
        unsigned int wrap = 0; /* bit mask: requests behind the disk head? */

        if (!rq1 || rq1 == rq2)
                return rq2;
        if (!rq2)
                return rq1;

        if (rq_is_sync(rq1) && !rq_is_sync(rq2))
                return rq1;
        else if (rq_is_sync(rq2) && !rq_is_sync(rq1))
                return rq2;
        if ((rq1->cmd_flags & REQ_META) && !(rq2->cmd_flags & REQ_META))
                return rq1;
        else if ((rq2->cmd_flags & REQ_META) && !(rq1->cmd_flags & REQ_META))
                return rq2;

        s1 = blk_rq_pos(rq1);
        s2 = blk_rq_pos(rq2);

        /*
         * By definition, 1KiB is 2 sectors.
         */
        back_max = bfqd->bfq_back_max * 2;

        /*
         * Strict one way elevator _except_ in the case where we allow
         * short backward seeks which are biased as twice the cost of a
         * similar forward seek.
         */
        if (s1 >= last)
                d1 = s1 - last;
        else if (s1 + back_max >= last)
                d1 = (last - s1) * bfqd->bfq_back_penalty;
        else
                wrap |= BFQ_RQ1_WRAP;

        if (s2 >= last)
                d2 = s2 - last;
        else if (s2 + back_max >= last)
                d2 = (last - s2) * bfqd->bfq_back_penalty;
        else
                wrap |= BFQ_RQ2_WRAP;

        /* Found required data */

        /*
         * By doing switch() on the bit mask "wrap" we avoid having to
         * check two variables for all permutations: --> faster!
         */
        switch (wrap) {
        case 0: /* common case for CFQ: rq1 and rq2 not wrapped */
                if (d1 < d2)
                        return rq1;
                else if (d2 < d1)
                        return rq2;

                if (s1 >= s2)
                        return rq1;
                else
                        return rq2;

        case BFQ_RQ2_WRAP:
                return rq1;
        case BFQ_RQ1_WRAP:
                return rq2;
        case BFQ_RQ1_WRAP|BFQ_RQ2_WRAP: /* both rqs wrapped */
        default:
                /*
                 * Since both rqs are wrapped,
                 * start with the one that's further behind head
                 * (--> only *one* back seek required),
                 * since back seek takes more time than forward.
                 */
                if (s1 <= s2)
                        return rq1;
                else
                        return rq2;
        }
}

/*
 * Async I/O can easily starve sync I/O (both sync reads and sync
 * writes), by consuming all tags. Similarly, storms of sync writes,
 * such as those that sync(2) may trigger, can starve sync reads.
 * Limit depths of async I/O and sync writes so as to counter both
 * problems.
 */
static void bfq_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
{
        struct bfq_data *bfqd = data->q->elevator->elevator_data;

        if (op_is_sync(op) && !op_is_write(op))
                return;

        data->shallow_depth =
                bfqd->word_depths[!!bfqd->wr_busy_queues][op_is_sync(op)];

        bfq_log(bfqd, "[%s] wr_busy %d sync %d depth %u",
                        __func__, bfqd->wr_busy_queues, op_is_sync(op),
                        data->shallow_depth);
}

static struct bfq_queue *
bfq_rq_pos_tree_lookup(struct bfq_data *bfqd, struct rb_root *root,
                     sector_t sector, struct rb_node **ret_parent,
                     struct rb_node ***rb_link)
{
        struct rb_node **p, *parent;
        struct bfq_queue *bfqq = NULL;

        parent = NULL;
        p = &root->rb_node;
        while (*p) {
                struct rb_node **n;

                parent = *p;
                bfqq = rb_entry(parent, struct bfq_queue, pos_node);

                /*
                 * Sort strictly based on sector. Smallest to the left,
                 * largest to the right.
                 */
                if (sector > blk_rq_pos(bfqq->next_rq))
                        n = &(*p)->rb_right;
                else if (sector < blk_rq_pos(bfqq->next_rq))
                        n = &(*p)->rb_left;
                else
                        break;
                p = n;
                bfqq = NULL;
        }

        *ret_parent = parent;
        if (rb_link)
                *rb_link = p;

        bfq_log(bfqd, "rq_pos_tree_lookup %llu: returning %d",
                (unsigned long long)sector,
                bfqq ? bfqq->pid : 0);

        return bfqq;
}

static bool bfq_too_late_for_merging(struct bfq_queue *bfqq)
{
        return bfqq->service_from_backlogged > 0 &&
                time_is_before_jiffies(bfqq->first_IO_time +
                                       bfq_merge_time_limit);
}

/*
 * The following function is not marked as __cold because it is
 * actually cold, but for the same performance goal described in the
 * comments on the likely() at the beginning of
 * bfq_setup_cooperator(). Unexpectedly, to reach an even lower
 * execution time for the case where this function is not invoked, we
 * had to add an unlikely() in each involved if().
 */
void __cold
bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq)
{
        struct rb_node **p, *parent;
        struct bfq_queue *__bfqq;

        if (bfqq->pos_root) {
                rb_erase(&bfqq->pos_node, bfqq->pos_root);
                bfqq->pos_root = NULL;
        }

        /* oom_bfqq does not participate in queue merging */
        if (bfqq == &bfqd->oom_bfqq)
                return;

        /*
         * bfqq cannot be merged any longer (see comments in
         * bfq_setup_cooperator): no point in adding bfqq into the
         * position tree.
         */
        if (bfq_too_late_for_merging(bfqq))
                return;

        if (bfq_class_idle(bfqq))
                return;
        if (!bfqq->next_rq)
                return;

        bfqq->pos_root = &bfq_bfqq_to_bfqg(bfqq)->rq_pos_tree;
        __bfqq = bfq_rq_pos_tree_lookup(bfqd, bfqq->pos_root,
                        blk_rq_pos(bfqq->next_rq), &parent, &p);
        if (!__bfqq) {
                rb_link_node(&bfqq->pos_node, parent, p);
                rb_insert_color(&bfqq->pos_node, bfqq->pos_root);
        } else
                bfqq->pos_root = NULL;
}

/*
 * The following function returns false either if every active queue
 * must receive the same share of the throughput (symmetric scenario),
 * or, as a special case, if bfqq must receive a share of the
 * throughput lower than or equal to the share that every other active
 * queue must receive.  If bfqq does sync I/O, then these are the only
 * two cases where bfqq happens to be guaranteed its share of the
 * throughput even if I/O dispatching is not plugged when bfqq remains
 * temporarily empty (for more details, see the comments in the
 * function bfq_better_to_idle()). For this reason, the return value
 * of this function is used to check whether I/O-dispatch plugging can
 * be avoided.
 *
 * The above first case (symmetric scenario) occurs when:
 * 1) all active queues have the same weight,
 * 2) all active queues belong to the same I/O-priority class,
 * 3) all active groups at the same level in the groups tree have the same
 *    weight,
 * 4) all active groups at the same level in the groups tree have the same
 *    number of children.
 *
 * Unfortunately, keeping the necessary state for evaluating exactly
 * the last two symmetry sub-conditions above would be quite complex
 * and time consuming. Therefore this function evaluates, instead,
 * only the following stronger three sub-conditions, for which it is
 * much easier to maintain the needed state:
 * 1) all active queues have the same weight,
 * 2) all active queues belong to the same I/O-priority class,
 * 3) there are no active groups.
 * In particular, the last condition is always true if hierarchical
 * support or the cgroups interface are not enabled, thus no state
 * needs to be maintained in this case.
 */
static bool bfq_asymmetric_scenario(struct bfq_data *bfqd,
                                   struct bfq_queue *bfqq)
{
        bool smallest_weight = bfqq &&
                bfqq->weight_counter &&
                bfqq->weight_counter ==
                container_of(
                        rb_first_cached(&bfqd->queue_weights_tree),
                        struct bfq_weight_counter,
                        weights_node);

        /*
         * For queue weights to differ, queue_weights_tree must contain
         * at least two nodes.
         */
        bool varied_queue_weights = !smallest_weight &&
                !RB_EMPTY_ROOT(&bfqd->queue_weights_tree.rb_root) &&
                (bfqd->queue_weights_tree.rb_root.rb_node->rb_left ||
                 bfqd->queue_weights_tree.rb_root.rb_node->rb_right);

        bool multiple_classes_busy =
                (bfqd->busy_queues[0] && bfqd->busy_queues[1]) ||
                (bfqd->busy_queues[0] && bfqd->busy_queues[2]) ||
                (bfqd->busy_queues[1] && bfqd->busy_queues[2]);

        return varied_queue_weights || multiple_classes_busy
#ifdef CONFIG_BFQ_GROUP_IOSCHED
               || bfqd->num_groups_with_pending_reqs > 0
#endif
                ;
}

/*
 * If the weight-counter tree passed as input contains no counter for
 * the weight of the input queue, then add that counter; otherwise just
 * increment the existing counter.
 *
 * Note that weight-counter trees contain few nodes in mostly symmetric
 * scenarios. For example, if all queues have the same weight, then the
 * weight-counter tree for the queues may contain at most one node.
 * This holds even if low_latency is on, because weight-raised queues
 * are not inserted in the tree.
 * In most scenarios, the rate at which nodes are created/destroyed
 * should be low too.
 */
void bfq_weights_tree_add(struct bfq_data *bfqd, struct bfq_queue *bfqq,
                          struct rb_root_cached *root)
{
        struct bfq_entity *entity = &bfqq->entity;
        struct rb_node **new = &(root->rb_root.rb_node), *parent = NULL;
        bool leftmost = true;

        /*
         * Do not insert if the queue is already associated with a
         * counter, which happens if:
         *   1) a request arrival has caused the queue to become both
         *      non-weight-raised, and hence change its weight, and
         *      backlogged; in this respect, each of the two events
         *      causes an invocation of this function,
         *   2) this is the invocation of this function caused by the
         *      second event. This second invocation is actually useless,
         *      and we handle this fact by exiting immediately. More
         *      efficient or clearer solutions might possibly be adopted.
         */
        if (bfqq->weight_counter)
                return;

        while (*new) {
                struct bfq_weight_counter *__counter = container_of(*new,
                                                struct bfq_weight_counter,
                                                weights_node);
                parent = *new;

                if (entity->weight == __counter->weight) {
                        bfqq->weight_counter = __counter;
                        goto inc_counter;
                }
                if (entity->weight < __counter->weight)
                        new = &((*new)->rb_left);
                else {
                        new = &((*new)->rb_right);
                        leftmost = false;
                }
        }

        bfqq->weight_counter = kzalloc(sizeof(struct bfq_weight_counter),
                                       GFP_ATOMIC);

        /*
         * In the unlucky event of an allocation failure, we just
         * exit. This will cause the weight of queue to not be
         * considered in bfq_asymmetric_scenario, which, in its turn,
         * causes the scenario to be deemed wrongly symmetric in case
         * bfqq's weight would have been the only weight making the
         * scenario asymmetric.  On the bright side, no unbalance will
         * however occur when bfqq becomes inactive again (the
         * invocation of this function is triggered by an activation
         * of queue).  In fact, bfq_weights_tree_remove does nothing
         * if !bfqq->weight_counter.
         */
        if (unlikely(!bfqq->weight_counter))
                return;

        bfqq->weight_counter->weight = entity->weight;
        rb_link_node(&bfqq->weight_counter->weights_node, parent, new);
        rb_insert_color_cached(&bfqq->weight_counter->weights_node, root,
                                leftmost);

inc_counter:
        bfqq->weight_counter->num_active++;
        bfqq->ref++;
}

/*
 * Decrement the weight counter associated with the queue, and, if the
 * counter reaches 0, remove the counter from the tree.
 * See the comments to the function bfq_weights_tree_add() for considerations
 * about overhead.
 */
void __bfq_weights_tree_remove(struct bfq_data *bfqd,
                               struct bfq_queue *bfqq,
                               struct rb_root_cached *root)
{
        if (!bfqq->weight_counter)
                return;

        bfqq->weight_counter->num_active--;
        if (bfqq->weight_counter->num_active > 0)
                goto reset_entity_pointer;

        rb_erase_cached(&bfqq->weight_counter->weights_node, root);
        kfree(bfqq->weight_counter);

reset_entity_pointer:
        bfqq->weight_counter = NULL;
        bfq_put_queue(bfqq);
}

/*
 * Invoke __bfq_weights_tree_remove on bfqq and decrement the number
 * of active groups for each queue's inactive parent entity.
 */
void bfq_weights_tree_remove(struct bfq_data *bfqd,
                             struct bfq_queue *bfqq)
{
        struct bfq_entity *entity = bfqq->entity.parent;

        for_each_entity(entity) {
                struct bfq_sched_data *sd = entity->my_sched_data;

                if (sd->next_in_service || sd->in_service_entity) {
                        /*
                         * entity is still active, because either
                         * next_in_service or in_service_entity is not
                         * NULL (see the comments on the definition of
                         * next_in_service for details on why
                         * in_service_entity must be checked too).
                         *
                         * As a consequence, its parent entities are
                         * active as well, and thus this loop must
                         * stop here.
                         */
                        break;
                }

                /*
                 * The decrement of num_groups_with_pending_reqs is
                 * not performed immediately upon the deactivation of
                 * entity, but it is delayed to when it also happens
                 * that the first leaf descendant bfqq of entity gets
                 * all its pending requests completed. The following
                 * instructions perform this delayed decrement, if
                 * needed. See the comments on
                 * num_groups_with_pending_reqs for details.
                 */
                if (entity->in_groups_with_pending_reqs) {
                        entity->in_groups_with_pending_reqs = false;
                        bfqd->num_groups_with_pending_reqs--;
                }
        }

        /*
         * Next function is invoked last, because it causes bfqq to be
         * freed if the following holds: bfqq is not in service and
         * has no dispatched request. DO NOT use bfqq after the next
         * function invocation.
         */
        __bfq_weights_tree_remove(bfqd, bfqq,
                                  &bfqd->queue_weights_tree);
}

/*
 * Return expired entry, or NULL to just start from scratch in rbtree.
 */
static struct request *bfq_check_fifo(struct bfq_queue *bfqq,
                                      struct request *last)
{
        struct request *rq;

        if (bfq_bfqq_fifo_expire(bfqq))
                return NULL;

        bfq_mark_bfqq_fifo_expire(bfqq);

        rq = rq_entry_fifo(bfqq->fifo.next);

        if (rq == last || ktime_get_ns() < rq->fifo_time)
                return NULL;

        bfq_log_bfqq(bfqq->bfqd, bfqq, "check_fifo: returned %p", rq);
        return rq;
}

static struct request *bfq_find_next_rq(struct bfq_data *bfqd,
                                        struct bfq_queue *bfqq,
                                        struct request *last)
{
        struct rb_node *rbnext = rb_next(&last->rb_node);
        struct rb_node *rbprev = rb_prev(&last->rb_node);
        struct request *next, *prev = NULL;

        /* Follow expired path, else get first next available. */
        next = bfq_check_fifo(bfqq, last);
        if (next)
                return next;

        if (rbprev)
                prev = rb_entry_rq(rbprev);

        if (rbnext)
                next = rb_entry_rq(rbnext);
        else {
                rbnext = rb_first(&bfqq->sort_list);
                if (rbnext && rbnext != &last->rb_node)
                        next = rb_entry_rq(rbnext);
        }

        return bfq_choose_req(bfqd, next, prev, blk_rq_pos(last));
}

/* see the definition of bfq_async_charge_factor for details */
static unsigned long bfq_serv_to_charge(struct request *rq,
                                        struct bfq_queue *bfqq)
{
        if (bfq_bfqq_sync(bfqq) || bfqq->wr_coeff > 1 ||
            bfq_asymmetric_scenario(bfqq->bfqd, bfqq))
                return blk_rq_sectors(rq);

        return blk_rq_sectors(rq) * bfq_async_charge_factor;
}

/**
 * bfq_updated_next_req - update the queue after a new next_rq selection.
 * @bfqd: the device data the queue belongs to.
 * @bfqq: the queue to update.
 *
 * If the first request of a queue changes we make sure that the queue
 * has enough budget to serve at least its first request (if the
 * request has grown).  We do this because if the queue has not enough
 * budget for its first request, it has to go through two dispatch
 * rounds to actually get it dispatched.
 */
static void bfq_updated_next_req(struct bfq_data *bfqd,
                                 struct bfq_queue *bfqq)
{
        struct bfq_entity *entity = &bfqq->entity;
        struct request *next_rq = bfqq->next_rq;
        unsigned long new_budget;

        if (!next_rq)
                return;

        if (bfqq == bfqd->in_service_queue)
                /*
                 * In order not to break guarantees, budgets cannot be
                 * changed after an entity has been selected.
                 */
                return;

        new_budget = max_t(unsigned long,
                           max_t(unsigned long, bfqq->max_budget,
                                 bfq_serv_to_charge(next_rq, bfqq)),
                           entity->service);
        if (entity->budget != new_budget) {
                entity->budget = new_budget;
                bfq_log_bfqq(bfqd, bfqq, "updated next rq: new budget %lu",
                                         new_budget);
                bfq_requeue_bfqq(bfqd, bfqq, false);
        }
}

static unsigned int bfq_wr_duration(struct bfq_data *bfqd)
{
        u64 dur;

        if (bfqd->bfq_wr_max_time > 0)
                return bfqd->bfq_wr_max_time;

        dur = bfqd->rate_dur_prod;
        do_div(dur, bfqd->peak_rate);

        /*
         * Limit duration between 3 and 25 seconds. The upper limit
         * has been conservatively set after the following worst case:
         * on a QEMU/KVM virtual machine
         * - running in a slow PC
         * - with a virtual disk stacked on a slow low-end 5400rpm HDD
         * - serving a heavy I/O workload, such as the sequential reading
         *   of several files
         * mplayer took 23 seconds to start, if constantly weight-raised.
         *
         * As for higher values than that accommodating the above bad
         * scenario, tests show that higher values would often yield
         * the opposite of the desired result, i.e., would worsen
         * responsiveness by allowing non-interactive applications to
         * preserve weight raising for too long.
         *
         * On the other end, lower values than 3 seconds make it
         * difficult for most interactive tasks to complete their jobs
         * before weight-raising finishes.
         */
        return clamp_val(dur, msecs_to_jiffies(3000), msecs_to_jiffies(25000));
}

/* switch back from soft real-time to interactive weight raising */
static void switch_back_to_interactive_wr(struct bfq_queue *bfqq,
                                          struct bfq_data *bfqd)
{
        bfqq->wr_coeff = bfqd->bfq_wr_coeff;
        bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);
        bfqq->last_wr_start_finish = bfqq->wr_start_at_switch_to_srt;
}

static void
bfq_bfqq_resume_state(struct bfq_queue *bfqq, struct bfq_data *bfqd,
                      struct bfq_io_cq *bic, bool bfq_already_existing)
{
        unsigned int old_wr_coeff = bfqq->wr_coeff;
        bool busy = bfq_already_existing && bfq_bfqq_busy(bfqq);

        if (bic->saved_has_short_ttime)
                bfq_mark_bfqq_has_short_ttime(bfqq);
        else
                bfq_clear_bfqq_has_short_ttime(bfqq);

        if (bic->saved_IO_bound)
                bfq_mark_bfqq_IO_bound(bfqq);
        else
                bfq_clear_bfqq_IO_bound(bfqq);

        bfqq->entity.new_weight = bic->saved_weight;
        bfqq->ttime = bic->saved_ttime;
        bfqq->wr_coeff = bic->saved_wr_coeff;
        bfqq->wr_start_at_switch_to_srt = bic->saved_wr_start_at_switch_to_srt;
        bfqq->last_wr_start_finish = bic->saved_last_wr_start_finish;
        bfqq->wr_cur_max_time = bic->saved_wr_cur_max_time;

        if (bfqq->wr_coeff > 1 && (bfq_bfqq_in_large_burst(bfqq) ||
            time_is_before_jiffies(bfqq->last_wr_start_finish +
                                   bfqq->wr_cur_max_time))) {
                if (bfqq->wr_cur_max_time == bfqd->bfq_wr_rt_max_time &&
                    !bfq_bfqq_in_large_burst(bfqq) &&
                    time_is_after_eq_jiffies(bfqq->wr_start_at_switch_to_srt +
                                             bfq_wr_duration(bfqd))) {
                        switch_back_to_interactive_wr(bfqq, bfqd);
                } else {
                        bfqq->wr_coeff = 1;
                        bfq_log_bfqq(bfqq->bfqd, bfqq,
                                     "resume state: switching off wr");
                }
        }

        /* make sure weight will be updated, however we got here */
        bfqq->entity.prio_changed = 1;

        if (likely(!busy))
                return;

        if (old_wr_coeff == 1 && bfqq->wr_coeff > 1)
                bfqd->wr_busy_queues++;
        else if (old_wr_coeff > 1 && bfqq->wr_coeff == 1)
                bfqd->wr_busy_queues--;
}

static int bfqq_process_refs(struct bfq_queue *bfqq)
{
        return bfqq->ref - bfqq->allocated - bfqq->entity.on_st_or_in_serv -
                (bfqq->weight_counter != NULL);
}

/* Empty burst list and add just bfqq (see comments on bfq_handle_burst) */
static void bfq_reset_burst_list(struct bfq_data *bfqd, struct bfq_queue *bfqq)
{
        struct bfq_queue *item;
        struct hlist_node *n;

        hlist_for_each_entry_safe(item, n, &bfqd->burst_list, burst_list_node)
                hlist_del_init(&item->burst_list_node);

        /*
         * Start the creation of a new burst list only if there is no
         * active queue. See comments on the conditional invocation of
         * bfq_handle_burst().
         */
        if (bfq_tot_busy_queues(bfqd) == 0) {
                hlist_add_head(&bfqq->burst_list_node, &bfqd->burst_list);
                bfqd->burst_size = 1;
        } else
                bfqd->burst_size = 0;

        bfqd->burst_parent_entity = bfqq->entity.parent;
}

/* Add bfqq to the list of queues in current burst (see bfq_handle_burst) */
static void bfq_add_to_burst(struct bfq_data *bfqd, struct bfq_queue *bfqq)
{
        /* Increment burst size to take into account also bfqq */
        bfqd->burst_size++;

        if (bfqd->burst_size == bfqd->bfq_large_burst_thresh) {
                struct bfq_queue *pos, *bfqq_item;
                struct hlist_node *n;

                /*
                 * Enough queues have been activated shortly after each
                 * other to consider this burst as large.
                 */
                bfqd->large_burst = true;

                /*
                 * We can now mark all queues in the burst list as
                 * belonging to a large burst.
                 */
                hlist_for_each_entry(bfqq_item, &bfqd->burst_list,
                                     burst_list_node)
                        bfq_mark_bfqq_in_large_burst(bfqq_item);
                bfq_mark_bfqq_in_large_burst(bfqq);

                /*
                 * From now on, and until the current burst finishes, any
                 * new queue being activated shortly after the last queue
                 * was inserted in the burst can be immediately marked as
                 * belonging to a large burst. So the burst list is not
                 * needed any more. Remove it.
                 */
                hlist_for_each_entry_safe(pos, n, &bfqd->burst_list,
                                          burst_list_node)
                        hlist_del_init(&pos->burst_list_node);
        } else /*
                * Burst not yet large: add bfqq to the burst list. Do
                * not increment the ref counter for bfqq, because bfqq
                * is removed from the burst list before freeing bfqq
                * in put_queue.
                */
                hlist_add_head(&bfqq->burst_list_node, &bfqd->burst_list);
}

/*
 * If many queues belonging to the same group happen to be created
 * shortly after each other, then the processes associated with these
 * queues have typically a common goal. In particular, bursts of queue
 * creations are usually caused by services or applications that spawn
 * many parallel threads/processes. Examples are systemd during boot,
 * or git grep. To help these processes get their job done as soon as
 * possible, it is usually better to not grant either weight-raising
 * or device idling to their queues, unless these queues must be
 * protected from the I/O flowing through other active queues.
 *
 * In this comment we describe, firstly, the reasons why this fact
 * holds, and, secondly, the next function, which implements the main
 * steps needed to properly mark these queues so that they can then be
 * treated in a different way.
 *
 * The above services or applications benefit mostly from a high
 * throughput: the quicker the requests of the activated queues are
 * cumulatively served, the sooner the target job of these queues gets
 * completed. As a consequence, weight-raising any of these queues,
 * which also implies idling the device for it, is almost always
 * counterproductive, unless there are other active queues to isolate
 * these new queues from. If there no other active queues, then
 * weight-raising these new queues just lowers throughput in most
 * cases.
 *
 * On the other hand, a burst of queue creations may be caused also by
 * the start of an application that does not consist of a lot of
 * parallel I/O-bound threads. In fact, with a complex application,
 * several short processes may need to be executed to start-up the
 * application. In this respect, to start an application as quickly as
 * possible, the best thing to do is in any case to privilege the I/O
 * related to the application with respect to all other
 * I/O. Therefore, the best strategy to start as quickly as possible
 * an application that causes a burst of queue creations is to
 * weight-raise all the queues created during the burst. This is the
 * exact opposite of the best strategy for the other type of bursts.
 *
 * In the end, to take the best action for each of the two cases, the
 * two types of bursts need to be distinguished. Fortunately, this
 * seems relatively easy, by looking at the sizes of the bursts. In
 * particular, we found a threshold such that only bursts with a
 * larger size than that threshold are apparently caused by
 * services or commands such as systemd or git grep. For brevity,
 * hereafter we call just 'large' these bursts. BFQ *does not*
 * weight-raise queues whose creation occurs in a large burst. In
 * addition, for each of these queues BFQ performs or does not perform
 * idling depending on which choice boosts the throughput more. The
 * exact choice depends on the device and request pattern at
 * hand.
 *
 * Unfortunately, false positives may occur while an interactive task
 * is starting (e.g., an application is being started). The
 * consequence is that the queues associated with the task do not
 * enjoy weight raising as expected. Fortunately these false positives
 * are very rare. They typically occur if some service happens to
 * start doing I/O exactly when the interactive task starts.
 *
 * Turning back to the next function, it is invoked only if there are
 * no active queues (apart from active queues that would belong to the
 * same, possible burst bfqq would belong to), and it implements all
 * the steps needed to detect the occurrence of a large burst and to
 * properly mark all the queues belonging to it (so that they can then
 * be treated in a different way). This goal is achieved by
 * maintaining a "burst list" that holds, temporarily, the queues that
 * belong to the burst in progress. The list is then used to mark
 * these queues as belonging to a large burst if the burst does become
 * large. The main steps are the following.
 *
 * . when the very first queue is created, the queue is inserted into the
 *   list (as it could be the first queue in a possible burst)
 *
 * . if the current burst has not yet become large, and a queue Q that does
 *   not yet belong to the burst is activated shortly after the last time
 *   at which a new queue entered the burst list, then the function appends
 *   Q to the burst list
 *
 * . if, as a consequence of the previous step, the burst size reaches
 *   the large-burst threshold, then
 *
 *     . all the queues in the burst list are marked as belonging to a
 *       large burst
 *
 *     . the burst list is deleted; in fact, the burst list already served
 *       its purpose (keeping temporarily track of the queues in a burst,
 *       so as to be able to mark them as belonging to a large burst in the
 *       previous sub-step), and now is not needed any more
 *
 *     . the device enters a large-burst mode
 *
 * . if a queue Q that does not belong to the burst is created while
 *   the device is in large-burst mode and shortly after the last time
 *   at which a queue either entered the burst list or was marked as
 *   belonging to the current large burst, then Q is immediately marked
 *   as belonging to a large burst.
 *
 * . if a queue Q that does not belong to the burst is created a while
 *   later, i.e., not shortly after, than the last time at which a queue
 *   either entered the burst list or was marked as belonging to the
 *   current large burst, then the current burst is deemed as finished and:
 *
 *        . the large-burst mode is reset if set
 *
 *        . the burst list is emptied
 *
 *        . Q is inserted in the burst list, as Q may be the first queue
 *          in a possible new burst (then the burst list contains just Q
 *          after this step).
 */
static void bfq_handle_burst(struct bfq_data *bfqd, struct bfq_queue *bfqq)
{
        /*
         * If bfqq is already in the burst list or is part of a large
         * burst, or finally has just been split, then there is
         * nothing else to do.
         */
        if (!hlist_unhashed(&bfqq->burst_list_node) ||
            bfq_bfqq_in_large_burst(bfqq) ||
            time_is_after_eq_jiffies(bfqq->split_time +
                                     msecs_to_jiffies(10)))
                return;

        /*
         * If bfqq's creation happens late enough, or bfqq belongs to
         * a different group than the burst group, then the current
         * burst is finished, and related data structures must be
         * reset.
         *
         * In this respect, consider the special case where bfqq is
         * the very first queue created after BFQ is selected for this
         * device. In this case, last_ins_in_burst and
         * burst_parent_entity are not yet significant when we get
         * here. But it is easy to verify that, whether or not the
         * following condition is true, bfqq will end up being
         * inserted into the burst list. In particular the list will
         * happen to contain only bfqq. And this is exactly what has
         * to happen, as bfqq may be the first queue of the first
         * burst.
         */
        if (time_is_before_jiffies(bfqd->last_ins_in_burst +
            bfqd->bfq_burst_interval) ||
            bfqq->entity.parent != bfqd->burst_parent_entity) {
                bfqd->large_burst = false;
                bfq_reset_burst_list(bfqd, bfqq);
                goto end;
        }

        /*
         * If we get here, then bfqq is being activated shortly after the
         * last queue. So, if the current burst is also large, we can mark
         * bfqq as belonging to this large burst immediately.
         */
        if (bfqd->large_burst) {
                bfq_mark_bfqq_in_large_burst(bfqq);
                goto end;
        }

        /*
         * If we get here, then a large-burst state has not yet been
         * reached, but bfqq is being activated shortly after the last
         * queue. Then we add bfqq to the burst.
         */
        bfq_add_to_burst(bfqd, bfqq);
end:
        /*
         * At this point, bfqq either has been added to the current
         * burst or has caused the current burst to terminate and a
         * possible new burst to start. In particular, in the second
         * case, bfqq has become the first queue in the possible new
         * burst.  In both cases last_ins_in_burst needs to be moved
         * forward.
         */
        bfqd->last_ins_in_burst = jiffies;
}

static int bfq_bfqq_budget_left(struct bfq_queue *bfqq)
{
        struct bfq_entity *entity = &bfqq->entity;

        return entity->budget - entity->service;
}

/*
 * If enough samples have been computed, return the current max budget
 * stored in bfqd, which is dynamically updated according to the
 * estimated disk peak rate; otherwise return the default max budget
 */
static int bfq_max_budget(struct bfq_data *bfqd)
{
        if (bfqd->budgets_assigned < bfq_stats_min_budgets)
                return bfq_default_max_budget;
        else
                return bfqd->bfq_max_budget;
}

/*
 * Return min budget, which is a fraction of the current or default
 * max budget (trying with 1/32)
 */
static int bfq_min_budget(struct bfq_data *bfqd)
{
        if (bfqd->budgets_assigned < bfq_stats_min_budgets)
                return bfq_default_max_budget / 32;
        else
                return bfqd->bfq_max_budget / 32;
}

/*
 * The next function, invoked after the input queue bfqq switches from
 * idle to busy, updates the budget of bfqq. The function also tells
 * whether the in-service queue should be expired, by returning
 * true. The purpose of expiring the in-service queue is to give bfqq
 * the chance to possibly preempt the in-service queue, and the reason
 * for preempting the in-service queue is to achieve one of the two
 * goals below.
 *
 * 1. Guarantee to bfqq its reserved bandwidth even if bfqq has
 * expired because it has remained idle. In particular, bfqq may have
 * expired for one of the following two reasons:
 *
 * - BFQQE_NO_MORE_REQUESTS bfqq did not enjoy any device idling
 *   and did not make it to issue a new request before its last
 *   request was served;
 *
 * - BFQQE_TOO_IDLE bfqq did enjoy device idling, but did not issue
 *   a new request before the expiration of the idling-time.
 *
 * Even if bfqq has expired for one of the above reasons, the process
 * associated with the queue may be however issuing requests greedily,
 * and thus be sensitive to the bandwidth it receives (bfqq may have
 * remained idle for other reasons: CPU high load, bfqq not enjoying
 * idling, I/O throttling somewhere in the path from the process to
 * the I/O scheduler, ...). But if, after every expiration for one of
 * the above two reasons, bfqq has to wait for the service of at least
 * one full budget of another queue before being served again, then
 * bfqq is likely to get a much lower bandwidth or resource time than
 * its reserved ones. To address this issue, two countermeasures need
 * to be taken.
 *
 * First, the budget and the timestamps of bfqq need to be updated in
 * a special way on bfqq reactivation: they need to be updated as if
 * bfqq did not remain idle and did not expire. In fact, if they are
 * computed as if bfqq expired and remained idle until reactivation,
 * then the process associated with bfqq is treated as if, instead of
 * being greedy, it stopped issuing requests when bfqq remained idle,
 * and restarts issuing requests only on this reactivation. In other
 * words, the scheduler does not help the process recover the "service
 * hole" between bfqq expiration and reactivation. As a consequence,
 * the process receives a lower bandwidth than its reserved one. In
 * contrast, to recover this hole, the budget must be updated as if
 * bfqq was not expired at all before this reactivation, i.e., it must
 * be set to the value of the remaining budget when bfqq was
 * expired. Along the same line, timestamps need to be assigned the
 * value they had the last time bfqq was selected for service, i.e.,
 * before last expiration. Thus timestamps need to be back-shifted
 * with respect to their normal computation (see [1] for more details
 * on this tricky aspect).
 *
 * Secondly, to allow the process to recover the hole, the in-service
 * queue must be expired too, to give bfqq the chance to preempt it
 * immediately. In fact, if bfqq has to wait for a full budget of the
 * in-service queue to be completed, then it may become impossible to
 * let the process recover the hole, even if the back-shifted
 * timestamps of bfqq are lower than those of the in-service queue. If
 * this happens for most or all of the holes, then the process may not
 * receive its reserved bandwidth. In this respect, it is worth noting
 * that, being the service of outstanding requests unpreemptible, a
 * little fraction of the holes may however be unrecoverable, thereby
 * causing a little loss of bandwidth.
 *
 * The last important point is detecting whether bfqq does need this
 * bandwidth recovery. In this respect, the next function deems the
 * process associated with bfqq greedy, and thus allows it to recover
 * the hole, if: 1) the process is waiting for the arrival of a new
 * request (which implies that bfqq expired for one of the above two
 * reasons), and 2) such a request has arrived soon. The first
 * condition is controlled through the flag non_blocking_wait_rq,
 * while the second through the flag arrived_in_time. If both
 * conditions hold, then the function computes the budget in the
 * above-described special way, and signals that the in-service queue
 * should be expired. Timestamp back-shifting is done later in
 * __bfq_activate_entity.
 *
 * 2. Reduce latency. Even if timestamps are not backshifted to let
 * the process associated with bfqq recover a service hole, bfqq may
 * however happen to have, after being (re)activated, a lower finish
 * timestamp than the in-service queue.  That is, the next budget of
 * bfqq may have to be completed before the one of the in-service
 * queue. If this is the case, then preempting the in-service queue
 * allows this goal to be achieved, apart from the unpreemptible,
 * outstanding requests mentioned above.
 *
 * Unfortunately, regardless of which of the above two goals one wants
 * to achieve, service trees need first to be updated to know whether
 * the in-service queue must be preempted. To have service trees
 * correctly updated, the in-service queue must be expired and
 * rescheduled, and bfqq must be scheduled too. This is one of the
 * most costly operations (in future versions, the scheduling
 * mechanism may be re-designed in such a way to make it possible to
 * know whether preemption is needed without needing to update service
 * trees). In addition, queue preemptions almost always cause random
 * I/O, which may in turn cause loss of throughput. Finally, there may
 * even be no in-service queue when the next function is invoked (so,
 * no queue to compare timestamps with). Because of these facts, the
 * next function adopts the following simple scheme to avoid costly
 * operations, too frequent preemptions and too many dependencies on
 * the state of the scheduler: it requests the expiration of the
 * in-service queue (unconditionally) only for queues that need to
 * recover a hole. Then it delegates to other parts of the code the
 * responsibility of handling the above case 2.
 */
static bool bfq_bfqq_update_budg_for_activation(struct bfq_data *bfqd,
                                                struct bfq_queue *bfqq,
                                                bool arrived_in_time)
{
        struct bfq_entity *entity = &bfqq->entity;

        /*
         * In the next compound condition, we check also whether there
         * is some budget left, because otherwise there is no point in
         * trying to go on serving bfqq with this same budget: bfqq
         * would be expired immediately after being selected for
         * service. This would only cause useless overhead.
         */
        if (bfq_bfqq_non_blocking_wait_rq(bfqq) && arrived_in_time &&
            bfq_bfqq_budget_left(bfqq) > 0) {
                /*
                 * We do not clear the flag non_blocking_wait_rq here, as
                 * the latter is used in bfq_activate_bfqq to signal
                 * that timestamps need to be back-shifted (and is
                 * cleared right after).
                 */

                /*
                 * In next assignment we rely on that either
                 * entity->service or entity->budget are not updated
                 * on expiration if bfqq is empty (see
                 * __bfq_bfqq_recalc_budget). Thus both quantities
                 * remain unchanged after such an expiration, and the
                 * following statement therefore assigns to
                 * entity->budget the remaining budget on such an
                 * expiration.
                 */
                entity->budget = min_t(unsigned long,
                                       bfq_bfqq_budget_left(bfqq),
                                       bfqq->max_budget);

                /*
                 * At this point, we have used entity->service to get
                 * the budget left (needed for updating
                 * entity->budget). Thus we finally can, and have to,
                 * reset entity->service. The latter must be reset
                 * because bfqq would otherwise be charged again for
                 * the service it has received during its previous
                 * service slot(s).
                 */
                entity->service = 0;

                return true;
        }

        /*
         * We can finally complete expiration, by setting service to 0.
         */
        entity->service = 0;
        entity->budget = max_t(unsigned long, bfqq->max_budget,
                               bfq_serv_to_charge(bfqq->next_rq, bfqq));
        bfq_clear_bfqq_non_blocking_wait_rq(bfqq);
        return false;
}

/*
 * Return the farthest past time instant according to jiffies
 * macros.
 */
static unsigned long bfq_smallest_from_now(void)
{
        return jiffies - MAX_JIFFY_OFFSET;
}

static void bfq_update_bfqq_wr_on_rq_arrival(struct bfq_data *bfqd,
                                             struct bfq_queue *bfqq,
                                             unsigned int old_wr_coeff,
                                             bool wr_or_deserves_wr,
                                             bool interactive,
                                             bool in_burst,
                                             bool soft_rt)
{
        if (old_wr_coeff == 1 && wr_or_deserves_wr) {
                /* start a weight-raising period */
                if (interactive) {
                        bfqq->service_from_wr = 0;
                        bfqq->wr_coeff = bfqd->bfq_wr_coeff;
                        bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);
                } else {
                        /*
                         * No interactive weight raising in progress
                         * here: assign minus infinity to
                         * wr_start_at_switch_to_srt, to make sure
                         * that, at the end of the soft-real-time
                         * weight raising periods that is starting
                         * now, no interactive weight-raising period
                         * may be wrongly considered as still in
                         * progress (and thus actually started by
                         * mistake).
                         */
                        bfqq->wr_start_at_switch_to_srt =
                                bfq_smallest_from_now();
                        bfqq->wr_coeff = bfqd->bfq_wr_coeff *
                                BFQ_SOFTRT_WEIGHT_FACTOR;
                        bfqq->wr_cur_max_time =
                                bfqd->bfq_wr_rt_max_time;
                }

                /*
                 * If needed, further reduce budget to make sure it is
                 * close to bfqq's backlog, so as to reduce the
                 * scheduling-error component due to a too large
                 * budget. Do not care about throughput consequences,
                 * but only about latency. Finally, do not assign a
                 * too small budget either, to avoid increasing
                 * latency by causing too frequent expirations.
                 */
                bfqq->entity.budget = min_t(unsigned long,
                                            bfqq->entity.budget,
                                            2 * bfq_min_budget(bfqd));
        } else if (old_wr_coeff > 1) {
                if (interactive) { /* update wr coeff and duration */
                        bfqq->wr_coeff = bfqd->bfq_wr_coeff;
                        bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);
                } else if (in_burst)
                        bfqq->wr_coeff = 1;
                else if (soft_rt) {
                        /*
                         * The application is now or still meeting the
                         * requirements for being deemed soft rt.  We
                         * can then correctly and safely (re)charge
                         * the weight-raising duration for the
                         * application with the weight-raising
                         * duration for soft rt applications.
                         *
                         * In particular, doing this recharge now, i.e.,
                         * before the weight-raising period for the
                         * application finishes, reduces the probability
                         * of the following negative scenario:
                         * 1) the weight of a soft rt application is
                         *    raised at startup (as for any newly
                         *    created application),
                         * 2) since the application is not interactive,
                         *    at a certain time weight-raising is
                         *    stopped for the application,
                         * 3) at that time the application happens to
                         *    still have pending requests, and hence
                         *    is destined to not have a chance to be
                         *    deemed soft rt before these requests are
                         *    completed (see the comments to the
                         *    function bfq_bfqq_softrt_next_start()
                         *    for details on soft rt detection),
                         * 4) these pending requests experience a high
                         *    latency because the application is not
                         *    weight-raised while they are pending.
                         */
                        if (bfqq->wr_cur_max_time !=
                                bfqd->bfq_wr_rt_max_time) {
                                bfqq->wr_start_at_switch_to_srt =
                                        bfqq->last_wr_start_finish;

                                bfqq->wr_cur_max_time =
                                        bfqd->bfq_wr_rt_max_time;
                                bfqq->wr_coeff = bfqd->bfq_wr_coeff *
                                        BFQ_SOFTRT_WEIGHT_FACTOR;
                        }
                        bfqq->last_wr_start_finish = jiffies;
                }
        }
}

static bool bfq_bfqq_idle_for_long_time(struct bfq_data *bfqd,
                                        struct bfq_queue *bfqq)
{
        return bfqq->dispatched == 0 &&
                time_is_before_jiffies(
                        bfqq->budget_timeout +
                        bfqd->bfq_wr_min_idle_time);
}


/*
 * Return true if bfqq is in a higher priority class, or has a higher
 * weight than the in-service queue.
 */
static bool bfq_bfqq_higher_class_or_weight(struct bfq_queue *bfqq,
                                            struct bfq_queue *in_serv_bfqq)
{
        int bfqq_weight, in_serv_weight;

        if (bfqq->ioprio_class < in_serv_bfqq->ioprio_class)
                return true;

        if (in_serv_bfqq->entity.parent == bfqq->entity.parent) {
                bfqq_weight = bfqq->entity.weight;
                in_serv_weight = in_serv_bfqq->entity.weight;
        } else {
                if (bfqq->entity.parent)
                        bfqq_weight = bfqq->entity.parent->weight;
                else
                        bfqq_weight = bfqq->entity.weight;
                if (in_serv_bfqq->entity.parent)
                        in_serv_weight = in_serv_bfqq->entity.parent->weight;
                else
                        in_serv_weight = in_serv_bfqq->entity.weight;
        }

        return bfqq_weight > in_serv_weight;
}

static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd,
                                             struct bfq_queue *bfqq,
                                             int old_wr_coeff,
                                             struct request *rq,
                                             bool *interactive)
{
        bool soft_rt, in_burst, wr_or_deserves_wr,
                bfqq_wants_to_preempt,
                idle_for_long_time = bfq_bfqq_idle_for_long_time(bfqd, bfqq),
                /*
                 * See the comments on
                 * bfq_bfqq_update_budg_for_activation for
                 * details on the usage of the next variable.
                 */
                arrived_in_time =  ktime_get_ns() <=
                        bfqq->ttime.last_end_request +
                        bfqd->bfq_slice_idle * 3;


        /*
         * bfqq deserves to be weight-raised if:
         * - it is sync,
         * - it does not belong to a large burst,
         * - it has been idle for enough time or is soft real-time,
         * - is linked to a bfq_io_cq (it is not shared in any sense).
         */
        in_burst = bfq_bfqq_in_large_burst(bfqq);
        soft_rt = bfqd->bfq_wr_max_softrt_rate > 0 &&
                !BFQQ_TOTALLY_SEEKY(bfqq) &&
                !in_burst &&
                time_is_before_jiffies(bfqq->soft_rt_next_start) &&
                bfqq->dispatched == 0;
        *interactive = !in_burst && idle_for_long_time;
        wr_or_deserves_wr = bfqd->low_latency &&
                (bfqq->wr_coeff > 1 ||
                 (bfq_bfqq_sync(bfqq) &&
                  bfqq->bic && (*interactive || soft_rt)));

        /*
         * Using the last flag, update budget and check whether bfqq
         * may want to preempt the in-service queue.
         */
        bfqq_wants_to_preempt =
                bfq_bfqq_update_budg_for_activation(bfqd, bfqq,
                                                    arrived_in_time);

        /*
         * If bfqq happened to be activated in a burst, but has been
         * idle for much more than an interactive queue, then we
         * assume that, in the overall I/O initiated in the burst, the
         * I/O associated with bfqq is finished. So bfqq does not need
         * to be treated as a queue belonging to a burst
         * anymore. Accordingly, we reset bfqq's in_large_burst flag
         * if set, and remove bfqq from the burst list if it's
         * there. We do not decrement burst_size, because the fact
         * that bfqq does not need to belong to the burst list any
         * more does not invalidate the fact that bfqq was created in
         * a burst.
         */
        if (likely(!bfq_bfqq_just_created(bfqq)) &&
            idle_for_long_time &&
            time_is_before_jiffies(
                    bfqq->budget_timeout +
                    msecs_to_jiffies(10000))) {
                hlist_del_init(&bfqq->burst_list_node);
                bfq_clear_bfqq_in_large_burst(bfqq);
        }

        bfq_clear_bfqq_just_created(bfqq);


        if (!bfq_bfqq_IO_bound(bfqq)) {
                if (arrived_in_time) {
                        bfqq->requests_within_timer++;
                        if (bfqq->requests_within_timer >=
                            bfqd->bfq_requests_within_timer)
                                bfq_mark_bfqq_IO_bound(bfqq);
                } else
                        bfqq->requests_within_timer = 0;
        }

        if (bfqd->low_latency) {
                if (unlikely(time_is_after_jiffies(bfqq->split_time)))
                        /* wraparound */
                        bfqq->split_time =
                                jiffies - bfqd->bfq_wr_min_idle_time - 1;

                if (time_is_before_jiffies(bfqq->split_time +
                                           bfqd->bfq_wr_min_idle_time)) {
                        bfq_update_bfqq_wr_on_rq_arrival(bfqd, bfqq,
                                                         old_wr_coeff,
                                                         wr_or_deserves_wr,
                                                         *interactive,
                                                         in_burst,
                                                         soft_rt);

                        if (old_wr_coeff != bfqq->wr_coeff)
                                bfqq->entity.prio_changed = 1;
                }
        }

        bfqq->last_idle_bklogged = jiffies;
        bfqq->service_from_backlogged = 0;
        bfq_clear_bfqq_softrt_update(bfqq);

        bfq_add_bfqq_busy(bfqd, bfqq);

        /*
         * Expire in-service queue only if preemption may be needed
         * for guarantees. In particular, we care only about two
         * cases. The first is that bfqq has to recover a service
         * hole, as explained in the comments on
         * bfq_bfqq_update_budg_for_activation(), i.e., that
         * bfqq_wants_to_preempt is true. However, if bfqq does not
         * carry time-critical I/O, then bfqq's bandwidth is less
         * important than that of queues that carry time-critical I/O.
         * So, as a further constraint, we consider this case only if
         * bfqq is at least as weight-raised, i.e., at least as time
         * critical, as the in-service queue.
         *
         * The second case is that bfqq is in a higher priority class,
         * or has a higher weight than the in-service queue. If this
         * condition does not hold, we don't care because, even if
         * bfqq does not start to be served immediately, the resulting
         * delay for bfqq's I/O is however lower or much lower than
         * the ideal completion time to be guaranteed to bfqq's I/O.
         *
         * In both cases, preemption is needed only if, according to
         * the timestamps of both bfqq and of the in-service queue,
         * bfqq actually is the next queue to serve. So, to reduce
         * useless preemptions, the return value of
         * next_queue_may_preempt() is considered in the next compound
         * condition too. Yet next_queue_may_preempt() just checks a
         * simple, necessary condition for bfqq to be the next queue
         * to serve. In fact, to evaluate a sufficient condition, the
         * timestamps of the in-service queue would need to be
         * updated, and this operation is quite costly (see the
         * comments on bfq_bfqq_update_budg_for_activation()).
         */
        if (bfqd->in_service_queue &&
            ((bfqq_wants_to_preempt &&
              bfqq->wr_coeff >= bfqd->in_service_queue->wr_coeff) ||
             bfq_bfqq_higher_class_or_weight(bfqq, bfqd->in_service_queue)) &&
            next_queue_may_preempt(bfqd))
                bfq_bfqq_expire(bfqd, bfqd->in_service_queue,
                                false, BFQQE_PREEMPTED);
}

static void bfq_reset_inject_limit(struct bfq_data *bfqd,
                                   struct bfq_queue *bfqq)
{
        /* invalidate baseline total service time */
        bfqq->last_serv_time_ns = 0;

        /*
         * Reset pointer in case we are waiting for
         * some request completion.
         */
        bfqd->waited_rq = NULL;

        /*
         * If bfqq has a short think time, then start by setting the
         * inject limit to 0 prudentially, because the service time of
         * an injected I/O request may be higher than the think time
         * of bfqq, and therefore, if one request was injected when
         * bfqq remains empty, this injected request might delay the
         * service of the next I/O request for bfqq significantly. In
         * case bfqq can actually tolerate some injection, then the
         * adaptive update will however raise the limit soon. This
         * lucky circumstance holds exactly because bfqq has a short
         * think time, and thus, after remaining empty, is likely to
         * get new I/O enqueued---and then completed---before being
         * expired. This is the very pattern that gives the
         * limit-update algorithm the chance to measure the effect of
         * injection on request service times, and then to update the
         * limit accordingly.
         *
         * However, in the following special case, the inject limit is
         * left to 1 even if the think time is short: bfqq's I/O is
         * synchronized with that of some other queue, i.e., bfqq may
         * receive new I/O only after the I/O of the other queue is
         * completed. Keeping the inject limit to 1 allows the
         * blocking I/O to be served while bfqq is in service. And
         * this is very convenient both for bfqq and for overall
         * throughput, as explained in detail in the comments in
         * bfq_update_has_short_ttime().
         *
         * On the opposite end, if bfqq has a long think time, then
         * start directly by 1, because:
         * a) on the bright side, keeping at most one request in
         * service in the drive is unlikely to cause any harm to the
         * latency of bfqq's requests, as the service time of a single
         * request is likely to be lower than the think time of bfqq;
         * b) on the downside, after becoming empty, bfqq is likely to
         * expire before getting its next request. With this request
         * arrival pattern, it is very hard to sample total service
         * times and update the inject limit accordingly (see comments
         * on bfq_update_inject_limit()). So the limit is likely to be
         * never, or at least seldom, updated.  As a consequence, by
         * setting the limit to 1, we avoid that no injection ever
         * occurs with bfqq. On the downside, this proactive step
         * further reduces chances to actually compute the baseline
         * total service time. Thus it reduces chances to execute the
         * limit-update algorithm and possibly raise the limit to more
         * than 1.
         */
        if (bfq_bfqq_has_short_ttime(bfqq))
                bfqq->inject_limit = 0;
        else
                bfqq->inject_limit = 1;

        bfqq->decrease_time_jif = jiffies;
}

static void bfq_add_request(struct request *rq)
{
        struct bfq_queue *bfqq = RQ_BFQQ(rq);
        struct bfq_data *bfqd = bfqq->bfqd;
        struct request *next_rq, *prev;
        unsigned int old_wr_coeff = bfqq->wr_coeff;
        bool interactive = false;

        bfq_log_bfqq(bfqd, bfqq, "add_request %d", rq_is_sync(rq));
        bfqq->queued[rq_is_sync(rq)]++;
        bfqd->queued++;

        if (RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_bfqq_sync(bfqq)) {
                /*
                 * Detect whether bfqq's I/O seems synchronized with
                 * that of some other queue, i.e., whether bfqq, after
                 * remaining empty, happens to receive new I/O only
                 * right after some I/O request of the other queue has
                 * been completed. We call waker queue the other
                 * queue, and we assume, for simplicity, that bfqq may
                 * have at most one waker queue.
                 *
                 * A remarkable throughput boost can be reached by
                 * unconditionally injecting the I/O of the waker
                 * queue, every time a new bfq_dispatch_request
                 * happens to be invoked while I/O is being plugged
                 * for bfqq.  In addition to boosting throughput, this
                 * unblocks bfqq's I/O, thereby improving bandwidth
                 * and latency for bfqq. Note that these same results
                 * may be achieved with the general injection
                 * mechanism, but less effectively. For details on
                 * this aspect, see the comments on the choice of the
                 * queue for injection in bfq_select_queue().
                 *
                 * Turning back to the detection of a waker queue, a
                 * queue Q is deemed as a waker queue for bfqq if, for
                 * two consecutive times, bfqq happens to become non
                 * empty right after a request of Q has been
                 * completed. In particular, on the first time, Q is
                 * tentatively set as a candidate waker queue, while
                 * on the second time, the flag
                 * bfq_bfqq_has_waker(bfqq) is set to confirm that Q
                 * is a waker queue for bfqq. These detection steps
                 * are performed only if bfqq has a long think time,
                 * so as to make it more likely that bfqq's I/O is
                 * actually being blocked by a synchronization. This
                 * last filter, plus the above two-times requirement,
                 * make false positives less likely.
                 *
                 * NOTE
                 *
                 * The sooner a waker queue is detected, the sooner
                 * throughput can be boosted by injecting I/O from the
                 * waker queue. Fortunately, detection is likely to be
                 * actually fast, for the following reasons. While
                 * blocked by synchronization, bfqq has a long think
                 * time. This implies that bfqq's inject limit is at
                 * least equal to 1 (see the comments in
                 * bfq_update_inject_limit()). So, thanks to
                 * injection, the waker queue is likely to be served
                 * during the very first I/O-plugging time interval
                 * for bfqq. This triggers the first step of the
                 * detection mechanism. Thanks again to injection, the
                 * candidate waker queue is then likely to be
                 * confirmed no later than during the next
                 * I/O-plugging interval for bfqq.
                 */
                if (bfqd->last_completed_rq_bfqq &&
                    !bfq_bfqq_has_short_ttime(bfqq) &&
                    ktime_get_ns() - bfqd->last_completion <
                    200 * NSEC_PER_USEC) {
                        if (bfqd->last_completed_rq_bfqq != bfqq &&
                            bfqd->last_completed_rq_bfqq !=
                            bfqq->waker_bfqq) {
                                /*
                                 * First synchronization detected with
                                 * a candidate waker queue, or with a
                                 * different candidate waker queue
                                 * from the current one.
                                 */
                                bfqq->waker_bfqq = bfqd->last_completed_rq_bfqq;

                                /*
                                 * If the waker queue disappears, then
                                 * bfqq->waker_bfqq must be reset. To
                                 * this goal, we maintain in each
                                 * waker queue a list, woken_list, of
                                 * all the queues that reference the
                                 * waker queue through their
                                 * waker_bfqq pointer. When the waker
                                 * queue exits, the waker_bfqq pointer
                                 * of all the queues in the woken_list
                                 * is reset.
                                 *
                                 * In addition, if bfqq is already in
                                 * the woken_list of a waker queue,
                                 * then, before being inserted into
                                 * the woken_list of a new waker
                                 * queue, bfqq must be removed from
                                 * the woken_list of the old waker
                                 * queue.
                                 */
                                if (!hlist_unhashed(&bfqq->woken_list_node))
                                        hlist_del_init(&bfqq->woken_list_node);
                                hlist_add_head(&bfqq->woken_list_node,
                                    &bfqd->last_completed_rq_bfqq->woken_list);

                                bfq_clear_bfqq_has_waker(bfqq);
                        } else if (bfqd->last_completed_rq_bfqq ==
                                   bfqq->waker_bfqq &&
                                   !bfq_bfqq_has_waker(bfqq)) {
                                /*
                                 * synchronization with waker_bfqq
                                 * seen for the second time
                                 */
                                bfq_mark_bfqq_has_waker(bfqq);
                        }
                }

                /*
                 * Periodically reset inject limit, to make sure that
                 * the latter eventually drops in case workload
                 * changes, see step (3) in the comments on
                 * bfq_update_inject_limit().
                 */
                if (time_is_before_eq_jiffies(bfqq->decrease_time_jif +
                                             msecs_to_jiffies(1000)))
                        bfq_reset_inject_limit(bfqd, bfqq);

                /*
                 * The following conditions must hold to setup a new
                 * sampling of total service time, and then a new
                 * update of the inject limit:
                 * - bfqq is in service, because the total service
                 *   time is evaluated only for the I/O requests of
                 *   the queues in service;
                 * - this is the right occasion to compute or to

                 *   lower the baseline total service time, because
                 *   there are actually no requests in the drive,
                 *   or
                 *   the baseline total service time is available, and
                 *   this is the right occasion to compute the other
                 *   quantity needed to update the inject limit, i.e.,
                 *   the total service time caused by the amount of
                 *   injection allowed by the current value of the
                 *   limit. It is the right occasion because injection
                 *   has actually been performed during the service
                 *   hole, and there are still in-flight requests,
                 *   which are very likely to be exactly the injected
                 *   requests, or part of them;
                 * - the minimum interval for sampling the total
                 *   service time and updating the inject limit has
                 *   elapsed.
                 */
                if (bfqq == bfqd->in_service_queue &&
                    (bfqd->rq_in_driver == 0 ||
                     (bfqq->last_serv_time_ns > 0 &&
                      bfqd->rqs_injected && bfqd->rq_in_driver > 0)) &&
                    time_is_before_eq_jiffies(bfqq->decrease_time_jif +
                                              msecs_to_jiffies(10))) {
                        bfqd->last_empty_occupied_ns = ktime_get_ns();
                        /*
                         * Start the state machine for measuring the
                         * total service time of rq: setting
                         * wait_dispatch will cause bfqd->waited_rq to
                         * be set when rq will be dispatched.
                         */
                        bfqd->wait_dispatch = true;
                        /*
                         * If there is no I/O in service in the drive,
                         * then possible injection occurred before the
                         * arrival of rq will not affect the total
                         * service time of rq. So the injection limit
                         * must not be updated as a function of such
                         * total service time, unless new injection
                         * occurs before rq is completed. To have the
                         * injection limit updated only in the latter
                         * case, reset rqs_injected here (rqs_injected
                         * will be set in case injection is performed
                         * on bfqq before rq is completed).
                         */
                        if (bfqd->rq_in_driver == 0)
                                bfqd->rqs_injected = false;
                }
        }

        elv_rb_add(&bfqq->sort_list, rq);

        /*
         * Check if this request is a better next-serve candidate.
         */
        prev = bfqq->next_rq;
        next_rq = bfq_choose_req(bfqd, bfqq->next_rq, rq, bfqd->last_position);
        bfqq->next_rq = next_rq;

        /*
         * Adjust priority tree position, if next_rq changes.
         * See comments on bfq_pos_tree_add_move() for the unlikely().
         */
        if (unlikely(!bfqd->nonrot_with_queueing && prev != bfqq->next_rq))
                bfq_pos_tree_add_move(bfqd, bfqq);

        if (!bfq_bfqq_busy(bfqq)) /* switching to busy ... */
                bfq_bfqq_handle_idle_busy_switch(bfqd, bfqq, old_wr_coeff,
                                                 rq, &interactive);
        else {
                if (bfqd->low_latency && old_wr_coeff == 1 && !rq_is_sync(rq) &&
                    time_is_before_jiffies(
                                bfqq->last_wr_start_finish +
                                bfqd->bfq_wr_min_inter_arr_async)) {
                        bfqq->wr_coeff = bfqd->bfq_wr_coeff;
                        bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);

                        bfqd->wr_busy_queues++;
                        bfqq->entity.prio_changed = 1;
                }
                if (prev != bfqq->next_rq)
                        bfq_updated_next_req(bfqd, bfqq);
        }

        /*
         * Assign jiffies to last_wr_start_finish in the following
         * cases:
         *
         * . if bfqq is not going to be weight-raised, because, for
         *   non weight-raised queues, last_wr_start_finish stores the
         *   arrival time of the last request; as of now, this piece
         *   of information is used only for deciding whether to
         *   weight-raise async queues
         *
         * . if bfqq is not weight-raised, because, if bfqq is now
         *   switching to weight-raised, then last_wr_start_finish
         *   stores the time when weight-raising starts
         *
         * . if bfqq is interactive, because, regardless of whether
         *   bfqq is currently weight-raised, the weight-raising
         *   period must start or restart (this case is considered
         *   separately because it is not detected by the above
         *   conditions, if bfqq is already weight-raised)
         *
         * last_wr_start_finish has to be updated also if bfqq is soft
         * real-time, because the weight-raising period is constantly
         * restarted on idle-to-busy transitions for these queues, but
         * this is already done in bfq_bfqq_handle_idle_busy_switch if
         * needed.
         */
        if (bfqd->low_latency &&
                (old_wr_coeff == 1 || bfqq->wr_coeff == 1 || interactive))
                bfqq->last_wr_start_finish = jiffies;
}

static struct request *bfq_find_rq_fmerge(struct bfq_data *bfqd,
                                          struct bio *bio,
                                          struct request_queue *q)
{
        struct bfq_queue *bfqq = bfqd->bio_bfqq;


        if (bfqq)
                return elv_rb_find(&bfqq->sort_list, bio_end_sector(bio));

        return NULL;
}

static sector_t get_sdist(sector_t last_pos, struct request *rq)
{
        if (last_pos)
                return abs(blk_rq_pos(rq) - last_pos);

        return 0;
}

#if 0 /* Still not clear if we can do without next two functions */
static void bfq_activate_request(struct request_queue *q, struct request *rq)
{
        struct bfq_data *bfqd = q->elevator->elevator_data;

        bfqd->rq_in_driver++;
}

static void bfq_deactivate_request(struct request_queue *q, struct request *rq)
{
        struct bfq_data *bfqd = q->elevator->elevator_data;

        bfqd->rq_in_driver--;
}
#endif

static void bfq_remove_request(struct request_queue *q,
                               struct request *rq)
{
        struct bfq_queue *bfqq = RQ_BFQQ(rq);
        struct bfq_data *bfqd = bfqq->bfqd;
        const int sync = rq_is_sync(rq);

        if (bfqq->next_rq == rq) {
                bfqq->next_rq = bfq_find_next_rq(bfqd, bfqq, rq);
                bfq_updated_next_req(bfqd, bfqq);
        }

        if (rq->queuelist.prev != &rq->queuelist)
                list_del_init(&rq->queuelist);
        bfqq->queued[sync]--;
        bfqd->queued--;
        elv_rb_del(&bfqq->sort_list, rq);

        elv_rqhash_del(q, rq);
        if (q->last_merge == rq)
                q->last_merge = NULL;

        if (RB_EMPTY_ROOT(&bfqq->sort_list)) {
                bfqq->next_rq = NULL;

                if (bfq_bfqq_busy(bfqq) && bfqq != bfqd->in_service_queue) {
                        bfq_del_bfqq_busy(bfqd, bfqq, false);
                        /*
                         * bfqq emptied. In normal operation, when
                         * bfqq is empty, bfqq->entity.service and
                         * bfqq->entity.budget must contain,
                         * respectively, the service received and the
                         * budget used last time bfqq emptied. These
                         * facts do not hold in this case, as at least
                         * this last removal occurred while bfqq is
                         * not in service. To avoid inconsistencies,
                         * reset both bfqq->entity.service and
                         * bfqq->entity.budget, if bfqq has still a
                         * process that may issue I/O requests to it.
                         */
                        bfqq->entity.budget = bfqq->entity.service = 0;
                }

                /*
                 * Remove queue from request-position tree as it is empty.
                 */
                if (bfqq->pos_root) {
                        rb_erase(&bfqq->pos_node, bfqq->pos_root);
                        bfqq->pos_root = NULL;
                }
        } else {
                /* see comments on bfq_pos_tree_add_move() for the unlikely() */
                if (unlikely(!bfqd->nonrot_with_queueing))
                        bfq_pos_tree_add_move(bfqd, bfqq);
        }

        if (rq->cmd_flags & REQ_META)
                bfqq->meta_pending--;

}

static bool bfq_bio_merge(struct blk_mq_hw_ctx *hctx, struct bio *bio,
                unsigned int nr_segs)
{
        struct request_queue *q = hctx->queue;
        struct bfq_data *bfqd = q->elevator->elevator_data;
        struct request *free = NULL;
        /*
         * bfq_bic_lookup grabs the queue_lock: invoke it now and
         * store its return value for later use, to avoid nesting
         * queue_lock inside the bfqd->lock. We assume that the bic
         * returned by bfq_bic_lookup does not go away before
         * bfqd->lock is taken.
         */
        struct bfq_io_cq *bic = bfq_bic_lookup(bfqd, current->io_context, q);
        bool ret;

        spin_lock_irq(&bfqd->lock);

        if (bic)
                bfqd->bio_bfqq = bic_to_bfqq(bic, op_is_sync(bio->bi_opf));
        else
                bfqd->bio_bfqq = NULL;
        bfqd->bio_bic = bic;

        ret = blk_mq_sched_try_merge(q, bio, nr_segs, &free);

        if (free)
                blk_mq_free_request(free);
        spin_unlock_irq(&bfqd->lock);

        return ret;
}

static int bfq_request_merge(struct request_queue *q, struct request **req,
                             struct bio *bio)
{
        struct bfq_data *bfqd = q->elevator->elevator_data;
        struct request *__rq;

        __rq = bfq_find_rq_fmerge(bfqd, bio, q);
        if (__rq && elv_bio_merge_ok(__rq, bio)) {
                *req = __rq;
                return ELEVATOR_FRONT_MERGE;
        }

        return ELEVATOR_NO_MERGE;
}

static struct bfq_queue *bfq_init_rq(struct request *rq);

static void bfq_request_merged(struct request_queue *q, struct request *req,
                               enum elv_merge type)
{
        if (type == ELEVATOR_FRONT_MERGE &&
            rb_prev(&req->rb_node) &&
            blk_rq_pos(req) <
            blk_rq_pos(container_of(rb_prev(&req->rb_node),
                                    struct request, rb_node))) {
                struct bfq_queue *bfqq = bfq_init_rq(req);
                struct bfq_data *bfqd;
                struct request *prev, *next_rq;

                if (!bfqq)
                        return;

                bfqd = bfqq->bfqd;

                /* Reposition request in its sort_list */
                elv_rb_del(&bfqq->sort_list, req);
                elv_rb_add(&bfqq->sort_list, req);

                /* Choose next request to be served for bfqq */
                prev = bfqq->next_rq;
                next_rq = bfq_choose_req(bfqd, bfqq->next_rq, req,
                                         bfqd->last_position);
                bfqq->next_rq = next_rq;
                /*
                 * If next_rq changes, update both the queue's budget to
                 * fit the new request and the queue's position in its
                 * rq_pos_tree.
                 */
                if (prev != bfqq->next_rq) {
                        bfq_updated_next_req(bfqd, bfqq);
                        /*
                         * See comments on bfq_pos_tree_add_move() for
                         * the unlikely().
                         */
                        if (unlikely(!bfqd->nonrot_with_queueing))
                                bfq_pos_tree_add_move(bfqd, bfqq);
                }
        }
}

/*
 * This function is called to notify the scheduler that the requests
 * rq and 'next' have been merged, with 'next' going away.  BFQ
 * exploits this hook to address the following issue: if 'next' has a
 * fifo_time lower that rq, then the fifo_time of rq must be set to
 * the value of 'next', to not forget the greater age of 'next'.
 *
 * NOTE: in this function we assume that rq is in a bfq_queue, basing
 * on that rq is picked from the hash table q->elevator->hash, which,
 * in its turn, is filled only with I/O requests present in
 * bfq_queues, while BFQ is in use for the request queue q. In fact,
 * the function that fills this hash table (elv_rqhash_add) is called
 * only by bfq_insert_request.
 */
static void bfq_requests_merged(struct request_queue *q, struct request *rq,
                                struct request *next)
{
        struct bfq_queue *bfqq = bfq_init_rq(rq),
                *next_bfqq = bfq_init_rq(next);

        if (!bfqq)
                return;

        /*
         * If next and rq belong to the same bfq_queue and next is older
         * than rq, then reposition rq in the fifo (by substituting next
         * with rq). Otherwise, if next and rq belong to different
         * bfq_queues, never reposition rq: in fact, we would have to
         * reposition it with respect to next's position in its own fifo,
         * which would most certainly be too expensive with respect to
         * the benefits.
         */
        if (bfqq == next_bfqq &&
            !list_empty(&rq->queuelist) && !list_empty(&next->queuelist) &&
            next->fifo_time < rq->fifo_time) {
                list_del_init(&rq->queuelist);
                list_replace_init(&next->queuelist, &rq->queuelist);
                rq->fifo_time = next->fifo_time;
        }

        if (bfqq->next_rq == next)
                bfqq->next_rq = rq;

        bfqg_stats_update_io_merged(bfqq_group(bfqq), next->cmd_flags);
}

/* Must be called with bfqq != NULL */
static void bfq_bfqq_end_wr(struct bfq_queue *bfqq)
{
        if (bfq_bfqq_busy(bfqq))
                bfqq->bfqd->wr_busy_queues--;
        bfqq->wr_coeff = 1;
        bfqq->wr_cur_max_time = 0;
        bfqq->last_wr_start_finish = jiffies;
        /*
         * Trigger a weight change on the next invocation of
         * __bfq_entity_update_weight_prio.
         */
        bfqq->entity.prio_changed = 1;
}

void bfq_end_wr_async_queues(struct bfq_data *bfqd,
                             struct bfq_group *bfqg)
{
        int i, j;

        for (i = 0; i < 2; i++)
                for (j = 0; j < IOPRIO_BE_NR; j++)
                        if (bfqg->async_bfqq[i][j])
                                bfq_bfqq_end_wr(bfqg->async_bfqq[i][j]);
        if (bfqg->async_idle_bfqq)
                bfq_bfqq_end_wr(bfqg->async_idle_bfqq);
}

static void bfq_end_wr(struct bfq_data *bfqd)
{
        struct bfq_queue *bfqq;

        spin_lock_irq(&bfqd->lock);

        list_for_each_entry(bfqq, &bfqd->active_list, bfqq_list)
                bfq_bfqq_end_wr(bfqq);
        list_for_each_entry(bfqq, &bfqd->idle_list, bfqq_list)
                bfq_bfqq_end_wr(bfqq);
        bfq_end_wr_async(bfqd);

        spin_unlock_irq(&bfqd->lock);
}

static sector_t bfq_io_struct_pos(void *io_struct, bool request)
{
        if (request)
                return blk_rq_pos(io_struct);
        else
                return ((struct bio *)io_struct)->bi_iter.bi_sector;
}

static int bfq_rq_close_to_sector(void *io_struct, bool request,
                                  sector_t sector)
{
        return abs(bfq_io_struct_pos(io_struct, request) - sector) <=
               BFQQ_CLOSE_THR;
}

static struct bfq_queue *bfqq_find_close(struct bfq_data *bfqd,
                                         struct bfq_queue *bfqq,
                                         sector_t sector)
{
        struct rb_root *root = &bfq_bfqq_to_bfqg(bfqq)->rq_pos_tree;
        struct rb_node *parent, *node;
        struct bfq_queue *__bfqq;

        if (RB_EMPTY_ROOT(root))
                return NULL;

        /*
         * First, if we find a request starting at the end of the last
         * request, choose it.
         */
        __bfqq = bfq_rq_pos_tree_lookup(bfqd, root, sector, &parent, NULL);
        if (__bfqq)
                return __bfqq;

        /*
         * If the exact sector wasn't found, the parent of the NULL leaf
         * will contain the closest sector (rq_pos_tree sorted by
         * next_request position).
         */
        __bfqq = rb_entry(parent, struct bfq_queue, pos_node);
        if (bfq_rq_close_to_sector(__bfqq->next_rq, true, sector))
                return __bfqq;

        if (blk_rq_pos(__bfqq->next_rq) < sector)
                node = rb_next(&__bfqq->pos_node);
        else
                node = rb_prev(&__bfqq->pos_node);
        if (!node)
                return NULL;

        __bfqq = rb_entry(node, struct bfq_queue, pos_node);
        if (bfq_rq_close_to_sector(__bfqq->next_rq, true, sector))
                return __bfqq;

        return NULL;
}

static struct bfq_queue *bfq_find_close_cooperator(struct bfq_data *bfqd,
                                                   struct bfq_queue *cur_bfqq,
                                                   sector_t sector)
{
        struct bfq_queue *bfqq;

        /*
         * We shall notice if some of the queues are cooperating,
         * e.g., working closely on the same area of the device. In
         * that case, we can group them together and: 1) don't waste
         * time idling, and 2) serve the union of their requests in
         * the best possible order for throughput.
         */
        bfqq = bfqq_find_close(bfqd, cur_bfqq, sector);
        if (!bfqq || bfqq == cur_bfqq)
                return NULL;

        return bfqq;
}

static struct bfq_queue *
bfq_setup_merge(struct bfq_queue *bfqq, struct bfq_queue *new_bfqq)
{
        int process_refs, new_process_refs;
        struct bfq_queue *__bfqq;

        /*
         * If there are no process references on the new_bfqq, then it is
         * unsafe to follow the ->new_bfqq chain as other bfqq's in the chain
         * may have dropped their last reference (not just their last process
         * reference).
         */
        if (!bfqq_process_refs(new_bfqq))
                return NULL;

        /* Avoid a circular list and skip interim queue merges. */
        while ((__bfqq = new_bfqq->new_bfqq)) {
                if (__bfqq == bfqq)
                        return NULL;
                new_bfqq = __bfqq;
        }

        process_refs = bfqq_process_refs(bfqq);
        new_process_refs = bfqq_process_refs(new_bfqq);
        /*
         * If the process for the bfqq has gone away, there is no
         * sense in merging the queues.
         */
        if (process_refs == 0 || new_process_refs == 0)
                return NULL;

        bfq_log_bfqq(bfqq->bfqd, bfqq, "scheduling merge with queue %d",
                new_bfqq->pid);

        /*
         * Merging is just a redirection: the requests of the process
         * owning one of the two queues are redirected to the other queue.
         * The latter queue, in its turn, is set as shared if this is the
         * first time that the requests of some process are redirected to
         * it.
         *
         * We redirect bfqq to new_bfqq and not the opposite, because
         * we are in the context of the process owning bfqq, thus we
         * have the io_cq of this process. So we can immediately
         * configure this io_cq to redirect the requests of the
         * process to new_bfqq. In contrast, the io_cq of new_bfqq is
         * not available any more (new_bfqq->bic == NULL).
         *
         * Anyway, even in case new_bfqq coincides with the in-service
         * queue, redirecting requests the in-service queue is the
         * best option, as we feed the in-service queue with new
         * requests close to the last request served and, by doing so,
         * are likely to increase the throughput.
         */
        bfqq->new_bfqq = new_bfqq;
        new_bfqq->ref += process_refs;
        return new_bfqq;
}

static bool bfq_may_be_close_cooperator(struct bfq_queue *bfqq,
                                        struct bfq_queue *new_bfqq)
{
        if (bfq_too_late_for_merging(new_bfqq))
                return false;

        if (bfq_class_idle(bfqq) || bfq_class_idle(new_bfqq) ||
            (bfqq->ioprio_class != new_bfqq->ioprio_class))
                return false;

        /*
         * If either of the queues has already been detected as seeky,
         * then merging it with the other queue is unlikely to lead to
         * sequential I/O.
         */
        if (BFQQ_SEEKY(bfqq) || BFQQ_SEEKY(new_bfqq))
                return false;

        /*
         * Interleaved I/O is known to be done by (some) applications
         * only for reads, so it does not make sense to merge async
         * queues.
         */
        if (!bfq_bfqq_sync(bfqq) || !bfq_bfqq_sync(new_bfqq))
                return false;

        return true;
}

/*
 * Attempt to schedule a merge of bfqq with the currently in-service
 * queue or with a close queue among the scheduled queues.  Return
 * NULL if no merge was scheduled, a pointer to the shared bfq_queue
 * structure otherwise.
 *
 * The OOM queue is not allowed to participate to cooperation: in fact, since
 * the requests temporarily redirected to the OOM queue could be redirected
 * again to dedicated queues at any time, the state needed to correctly
 * handle merging with the OOM queue would be quite complex and expensive
 * to maintain. Besides, in such a critical condition as an out of memory,
 * the benefits of queue merging may be little relevant, or even negligible.
 *
 * WARNING: queue merging may impair fairness among non-weight raised
 * queues, for at least two reasons: 1) the original weight of a
 * merged queue may change during the merged state, 2) even being the
 * weight the same, a merged queue may be bloated with many more
 * requests than the ones produced by its originally-associated
 * process.
 */
static struct bfq_queue *
bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
                     void *io_struct, bool request)
{
        struct bfq_queue *in_service_bfqq, *new_bfqq;

        /*
         * Do not perform queue merging if the device is non
         * rotational and performs internal queueing. In fact, such a
         * device reaches a high speed through internal parallelism
         * and pipelining. This means that, to reach a high
         * throughput, it must have many requests enqueued at the same
         * time. But, in this configuration, the internal scheduling
         * algorithm of the device does exactly the job of queue
         * merging: it reorders requests so as to obtain as much as
         * possible a sequential I/O pattern. As a consequence, with
         * the workload generated by processes doing interleaved I/O,
         * the throughput reached by the device is likely to be the
         * same, with and without queue merging.
         *
         * Disabling merging also provides a remarkable benefit in
         * terms of throughput. Merging tends to make many workloads
         * artificially more uneven, because of shared queues
         * remaining non empty for incomparably more time than
         * non-merged queues. This may accentuate workload
         * asymmetries. For example, if one of the queues in a set of
         * merged queues has a higher weight than a normal queue, then
         * the shared queue may inherit such a high weight and, by
         * staying almost always active, may force BFQ to perform I/O
         * plugging most of the time. This evidently makes it harder
         * for BFQ to let the device reach a high throughput.
         *
         * Finally, the likely() macro below is not used because one
         * of the two branches is more likely than the other, but to
         * have the code path after the following if() executed as
         * fast as possible for the case of a non rotational device
         * with queueing. We want it because this is the fastest kind
         * of device. On the opposite end, the likely() may lengthen
         * the execution time of BFQ for the case of slower devices
         * (rotational or at least without queueing). But in this case
         * the execution time of BFQ matters very little, if not at
         * all.
         */
        if (likely(bfqd->nonrot_with_queueing))
                return NULL;

        /*
         * Prevent bfqq from being merged if it has been created too
         * long ago. The idea is that true cooperating processes, and
         * thus their associated bfq_queues, are supposed to be
         * created shortly after each other. This is the case, e.g.,
         * for KVM/QEMU and dump I/O threads. Basing on this
         * assumption, the following filtering greatly reduces the
         * probability that two non-cooperating processes, which just
         * happen to do close I/O for some short time interval, have
         * their queues merged by mistake.
         */
        if (bfq_too_late_for_merging(bfqq))
                return NULL;

        if (bfqq->new_bfqq)
                return bfqq->new_bfqq;

        if (!io_struct || unlikely(bfqq == &bfqd->oom_bfqq))
                return NULL;

        /* If there is only one backlogged queue, don't search. */
        if (bfq_tot_busy_queues(bfqd) == 1)
                return NULL;

        in_service_bfqq = bfqd->in_service_queue;

        if (in_service_bfqq && in_service_bfqq != bfqq &&
            likely(in_service_bfqq != &bfqd->oom_bfqq) &&
            bfq_rq_close_to_sector(io_struct, request,
                                   bfqd->in_serv_last_pos) &&
            bfqq->entity.parent == in_service_bfqq->entity.parent &&
            bfq_may_be_close_cooperator(bfqq, in_service_bfqq)) {
                new_bfqq = bfq_setup_merge(bfqq, in_service_bfqq);
                if (new_bfqq)
                        return new_bfqq;
        }
        /*
         * Check whether there is a cooperator among currently scheduled
         * queues. The only thing we need is that the bio/request is not
         * NULL, as we need it to establish whether a cooperator exists.
         */
        new_bfqq = bfq_find_close_cooperator(bfqd, bfqq,
                        bfq_io_struct_pos(io_struct, request));

        if (new_bfqq && likely(new_bfqq != &bfqd->oom_bfqq) &&
            bfq_may_be_close_cooperator(bfqq, new_bfqq))
                return bfq_setup_merge(bfqq, new_bfqq);

        return NULL;
}

static void bfq_bfqq_save_state(struct bfq_queue *bfqq)
{
        struct bfq_io_cq *bic = bfqq->bic;

        /*
         * If !bfqq->bic, the queue is already shared or its requests
         * have already been redirected to a shared queue; both idle window
         * and weight raising state have already been saved. Do nothing.
         */
        if (!bic)
                return;

        bic->saved_weight = bfqq->entity.orig_weight;
        bic->saved_ttime = bfqq->ttime;
        bic->saved_has_short_ttime = bfq_bfqq_has_short_ttime(bfqq);
        bic->saved_IO_bound = bfq_bfqq_IO_bound(bfqq);
        bic->saved_in_large_burst = bfq_bfqq_in_large_burst(bfqq);
        bic->was_in_burst_list = !hlist_unhashed(&bfqq->burst_list_node);
        if (unlikely(bfq_bfqq_just_created(bfqq) &&
                     !bfq_bfqq_in_large_burst(bfqq) &&
                     bfqq->bfqd->low_latency)) {
                /*
                 * bfqq being merged right after being created: bfqq
                 * would have deserved interactive weight raising, but
                 * did not make it to be set in a weight-raised state,
                 * because of this early merge. Store directly the
                 * weight-raising state that would have been assigned
                 * to bfqq, so that to avoid that bfqq unjustly fails
                 * to enjoy weight raising if split soon.
                 */
                bic->saved_wr_coeff = bfqq->bfqd->bfq_wr_coeff;
                bic->saved_wr_start_at_switch_to_srt = bfq_smallest_from_now();
                bic->saved_wr_cur_max_time = bfq_wr_duration(bfqq->bfqd);
                bic->saved_last_wr_start_finish = jiffies;
        } else {
                bic->saved_wr_coeff = bfqq->wr_coeff;
                bic->saved_wr_start_at_switch_to_srt =
                        bfqq->wr_start_at_switch_to_srt;
                bic->saved_last_wr_start_finish = bfqq->last_wr_start_finish;
                bic->saved_wr_cur_max_time = bfqq->wr_cur_max_time;
        }
}

void bfq_release_process_ref(struct bfq_data *bfqd, struct bfq_queue *bfqq)
{
        /*
         * To prevent bfqq's service guarantees from being violated,
         * bfqq may be left busy, i.e., queued for service, even if
         * empty (see comments in __bfq_bfqq_expire() for
         * details). But, if no process will send requests to bfqq any
         * longer, then there is no point in keeping bfqq queued for
         * service. In addition, keeping bfqq queued for service, but
         * with no process ref any longer, may have caused bfqq to be
         * freed when dequeued from service. But this is assumed to
         * never happen.
         */
        if (bfq_bfqq_busy(bfqq) && RB_EMPTY_ROOT(&bfqq->sort_list) &&
            bfqq != bfqd->in_service_queue)
                bfq_del_bfqq_busy(bfqd, bfqq, false);

        bfq_put_queue(bfqq);
}

static void
bfq_merge_bfqqs(struct bfq_data *bfqd, struct bfq_io_cq *bic,
                struct bfq_queue *bfqq, struct bfq_queue *new_bfqq)
{
        bfq_log_bfqq(bfqd, bfqq, "merging with queue %lu",
                (unsigned long)new_bfqq->pid);
        /* Save weight raising and idle window of the merged queues */
        bfq_bfqq_save_state(bfqq);
        bfq_bfqq_save_state(new_bfqq);
        if (bfq_bfqq_IO_bound(bfqq))
                bfq_mark_bfqq_IO_bound(new_bfqq);
        bfq_clear_bfqq_IO_bound(bfqq);

        /*
         * If bfqq is weight-raised, then let new_bfqq inherit
         * weight-raising. To reduce false positives, neglect the case
         * where bfqq has just been created, but has not yet made it
         * to be weight-raised (which may happen because EQM may merge
         * bfqq even before bfq_add_request is executed for the first
         * time for bfqq). Handling this case would however be very
         * easy, thanks to the flag just_created.
         */
        if (new_bfqq->wr_coeff == 1 && bfqq->wr_coeff > 1) {
                new_bfqq->wr_coeff = bfqq->wr_coeff;
                new_bfqq->wr_cur_max_time = bfqq->wr_cur_max_time;
                new_bfqq->last_wr_start_finish = bfqq->last_wr_start_finish;
                new_bfqq->wr_start_at_switch_to_srt =
                        bfqq->wr_start_at_switch_to_srt;
                if (bfq_bfqq_busy(new_bfqq))
                        bfqd->wr_busy_queues++;
                new_bfqq->entity.prio_changed = 1;
        }

        if (bfqq->wr_coeff > 1) { /* bfqq has given its wr to new_bfqq */
                bfqq->wr_coeff = 1;
                bfqq->entity.prio_changed = 1;
                if (bfq_bfqq_busy(bfqq))
                        bfqd->wr_busy_queues--;
        }

        bfq_log_bfqq(bfqd, new_bfqq, "merge_bfqqs: wr_busy %d",
                     bfqd->wr_busy_queues);

        /*
         * Merge queues (that is, let bic redirect its requests to new_bfqq)
         */
        bic_set_bfqq(bic, new_bfqq, 1);
        bfq_mark_bfqq_coop(new_bfqq);
        /*
         * new_bfqq now belongs to at least two bics (it is a shared queue):
         * set new_bfqq->bic to NULL. bfqq either:
         * - does not belong to any bic any more, and hence bfqq->bic must
         *   be set to NULL, or
         * - is a queue whose owning bics have already been redirected to a
         *   different queue, hence the queue is destined to not belong to
         *   any bic soon and bfqq->bic is already NULL (therefore the next
         *   assignment causes no harm).
         */
        new_bfqq->bic = NULL;
        /*
         * If the queue is shared, the pid is the pid of one of the associated
         * processes. Which pid depends on the exact sequence of merge events
         * the queue underwent. So printing such a pid is useless and confusing
         * because it reports a random pid between those of the associated
         * processes.
         * We mark such a queue with a pid -1, and then print SHARED instead of
         * a pid in logging messages.
         */
        new_bfqq->pid = -1;
        bfqq->bic = NULL;
        bfq_release_process_ref(bfqd, bfqq);
}

static bool bfq_allow_bio_merge(struct request_queue *q, struct request *rq,
                                struct bio *bio)
{
        struct bfq_data *bfqd = q->elevator->elevator_data;
        bool is_sync = op_is_sync(bio->bi_opf);
        struct bfq_queue *bfqq = bfqd->bio_bfqq, *new_bfqq;

        /*
         * Disallow merge of a sync bio into an async request.
         */
        if (is_sync && !rq_is_sync(rq))
                return false;

        /*
         * Lookup the bfqq that this bio will be queued with. Allow
         * merge only if rq is queued there.
         */
        if (!bfqq)
                return false;

        /*
         * We take advantage of this function to perform an early merge
         * of the queues of possible cooperating processes.
         */
        new_bfqq = bfq_setup_cooperator(bfqd, bfqq, bio, false);
        if (new_bfqq) {
                /*
                 * bic still points to bfqq, then it has not yet been
                 * redirected to some other bfq_queue, and a queue
                 * merge between bfqq and new_bfqq can be safely
                 * fulfilled, i.e., bic can be redirected to new_bfqq
                 * and bfqq can be put.
                 */
                bfq_merge_bfqqs(bfqd, bfqd->bio_bic, bfqq,
                                new_bfqq);
                /*
                 * If we get here, bio will be queued into new_queue,
                 * so use new_bfqq to decide whether bio and rq can be
                 * merged.
                 */
                bfqq = new_bfqq;

                /*
                 * Change also bqfd->bio_bfqq, as
                 * bfqd->bio_bic now points to new_bfqq, and
                 * this function may be invoked again (and then may
                 * use again bqfd->bio_bfqq).
                 */
                bfqd->bio_bfqq = bfqq;
        }

        return bfqq == RQ_BFQQ(rq);
}

/*
 * Set the maximum time for the in-service queue to consume its
 * budget. This prevents seeky processes from lowering the throughput.
 * In practice, a time-slice service scheme is used with seeky
 * processes.
 */
static void bfq_set_budget_timeout(struct bfq_data *bfqd,
                                   struct bfq_queue *bfqq)
{
        unsigned int timeout_coeff;

        if (bfqq->wr_cur_max_time == bfqd->bfq_wr_rt_max_time)
                timeout_coeff = 1;
        else
                timeout_coeff = bfqq->entity.weight / bfqq->entity.orig_weight;

        bfqd->last_budget_start = ktime_get();

        bfqq->budget_timeout = jiffies +
                bfqd->bfq_timeout * timeout_coeff;
}

static void __bfq_set_in_service_queue(struct bfq_data *bfqd,
                                       struct bfq_queue *bfqq)
{
        if (bfqq) {
                bfq_clear_bfqq_fifo_expire(bfqq);

                bfqd->budgets_assigned = (bfqd->budgets_assigned * 7 + 256) / 8;

                if (time_is_before_jiffies(bfqq->last_wr_start_finish) &&
                    bfqq->wr_coeff > 1 &&
                    bfqq->wr_cur_max_time == bfqd->bfq_wr_rt_max_time &&
                    time_is_before_jiffies(bfqq->budget_timeout)) {
                        /*
                         * For soft real-time queues, move the start
                         * of the weight-raising period forward by the
                         * time the queue has not received any
                         * service. Otherwise, a relatively long
                         * service delay is likely to cause the
                         * weight-raising period of the queue to end,
                         * because of the short duration of the
                         * weight-raising period of a soft real-time
                         * queue.  It is worth noting that this move
                         * is not so dangerous for the other queues,
                         * because soft real-time queues are not
                         * greedy.
                         *
                         * To not add a further variable, we use the
                         * overloaded field budget_timeout to
                         * determine for how long the queue has not
                         * received service, i.e., how much time has
                         * elapsed since the queue expired. However,
                         * this is a little imprecise, because
                         * budget_timeout is set to jiffies if bfqq
                         * not only expires, but also remains with no
                         * request.
                         */
                        if (time_after(bfqq->budget_timeout,
                                       bfqq->last_wr_start_finish))
                                bfqq->last_wr_start_finish +=
                                        jiffies - bfqq->budget_timeout;
                        else
                                bfqq->last_wr_start_finish = jiffies;
                }

                bfq_set_budget_timeout(bfqd, bfqq);
                bfq_log_bfqq(bfqd, bfqq,
                             "set_in_service_queue, cur-budget = %d",
                             bfqq->entity.budget);
        }

        bfqd->in_service_queue = bfqq;
}

/*
 * Get and set a new queue for service.
 */
static struct bfq_queue *bfq_set_in_service_queue(struct bfq_data *bfqd)
{
        struct bfq_queue *bfqq = bfq_get_next_queue(bfqd);

        __bfq_set_in_service_queue(bfqd, bfqq);
        return bfqq;
}

static void bfq_arm_slice_timer(struct bfq_data *bfqd)
{
        struct bfq_queue *bfqq = bfqd->in_service_queue;
        u32 sl;

        bfq_mark_bfqq_wait_request(bfqq);

        /*
         * We don't want to idle for seeks, but we do want to allow
         * fair distribution of slice time for a process doing back-to-back
         * seeks. So allow a little bit of time for him to submit a new rq.
         */
        sl = bfqd->bfq_slice_idle;
        /*
         * Unless the queue is being weight-raised or the scenario is
         * asymmetric, grant only minimum idle time if the queue
         * is seeky. A long idling is preserved for a weight-raised
         * queue, or, more in general, in an asymmetric scenario,
         * because a long idling is needed for guaranteeing to a queue
         * its reserved share of the throughput (in particular, it is
         * needed if the queue has a higher weight than some other
         * queue).
         */
        if (BFQQ_SEEKY(bfqq) && bfqq->wr_coeff == 1 &&
            !bfq_asymmetric_scenario(bfqd, bfqq))
                sl = min_t(u64, sl, BFQ_MIN_TT);
        else if (bfqq->wr_coeff > 1)
                sl = max_t(u32, sl, 20ULL * NSEC_PER_MSEC);

        bfqd->last_idling_start = ktime_get();
        bfqd->last_idling_start_jiffies = jiffies;

        hrtimer_start(&bfqd->idle_slice_timer, ns_to_ktime(sl),
                      HRTIMER_MODE_REL);
        bfqg_stats_set_start_idle_time(bfqq_group(bfqq));
}

/*
 * In autotuning mode, max_budget is dynamically recomputed as the
 * amount of sectors transferred in timeout at the estimated peak
 * rate. This enables BFQ to utilize a full timeslice with a full
 * budget, even if the in-service queue is served at peak rate. And
 * this maximises throughput with sequential workloads.
 */
static unsigned long bfq_calc_max_budget(struct bfq_data *bfqd)
{
        return (u64)bfqd->peak_rate * USEC_PER_MSEC *
                jiffies_to_msecs(bfqd->bfq_timeout)>>BFQ_RATE_SHIFT;

}

/*
 * Update parameters related to throughput and responsiveness, as a
 * function of the estimated peak rate. See comments on
 * bfq_calc_max_budget(), and on the ref_wr_duration array.
 */
static void update_thr_responsiveness_params(struct bfq_data *bfqd)
{
        if (bfqd->bfq_user_max_budget == 0) {
                bfqd->bfq_max_budget =
                        bfq_calc_max_budget(bfqd);
                bfq_log(bfqd, "new max_budget = %d", bfqd->bfq_max_budget);
        }
}

static void bfq_reset_rate_computation(struct bfq_data *bfqd,
                                       struct request *rq)
{
        if (rq != NULL) { /* new rq dispatch now, reset accordingly */
                bfqd->last_dispatch = bfqd->first_dispatch = ktime_get_ns();
                bfqd->peak_rate_samples = 1;
                bfqd->sequential_samples = 0;
                bfqd->tot_sectors_dispatched = bfqd->last_rq_max_size =
                        blk_rq_sectors(rq);
        } else /* no new rq dispatched, just reset the number of samples */
                bfqd->peak_rate_samples = 0; /* full re-init on next disp. */

        bfq_log(bfqd,
                "reset_rate_computation at end, sample %u/%u tot_sects %llu",
                bfqd->peak_rate_samples, bfqd->sequential_samples,
                bfqd->tot_sectors_dispatched);
}

static void bfq_update_rate_reset(struct bfq_data *bfqd, struct request *rq)
{
        u32 rate, weight, divisor;

        /*
         * For the convergence property to hold (see comments on
         * bfq_update_peak_rate()) and for the assessment to be
         * reliable, a minimum number of samples must be present, and
         * a minimum amount of time must have elapsed. If not so, do
         * not compute new rate. Just reset parameters, to get ready
         * for a new evaluation attempt.
         */
        if (bfqd->peak_rate_samples < BFQ_RATE_MIN_SAMPLES ||
            bfqd->delta_from_first < BFQ_RATE_MIN_INTERVAL)
                goto reset_computation;

        /*
         * If a new request completion has occurred after last
         * dispatch, then, to approximate the rate at which requests
         * have been served by the device, it is more precise to
         * extend the observation interval to the last completion.
         */
        bfqd->delta_from_first =
                max_t(u64, bfqd->delta_from_first,
                      bfqd->last_completion - bfqd->first_dispatch);

        /*
         * Rate computed in sects/usec, and not sects/nsec, for
         * precision issues.
         */
        rate = div64_ul(bfqd->tot_sectors_dispatched<<BFQ_RATE_SHIFT,
                        div_u64(bfqd->delta_from_first, NSEC_PER_USEC));

        /*
         * Peak rate not updated if:
         * - the percentage of sequential dispatches is below 3/4 of the
         *   total, and rate is below the current estimated peak rate
         * - rate is unreasonably high (> 20M sectors/sec)
         */
        if ((bfqd->sequential_samples < (3 * bfqd->peak_rate_samples)>>2 &&
             rate <= bfqd->peak_rate) ||
                rate > 20<<BFQ_RATE_SHIFT)
                goto reset_computation;

        /*
         * We have to update the peak rate, at last! To this purpose,
         * we use a low-pass filter. We compute the smoothing constant
         * of the filter as a function of the 'weight' of the new
         * measured rate.
         *
         * As can be seen in next formulas, we define this weight as a
         * quantity proportional to how sequential the workload is,
         * and to how long the observation time interval is.
         *
         * The weight runs from 0 to 8. The maximum value of the
         * weight, 8, yields the minimum value for the smoothing
         * constant. At this minimum value for the smoothing constant,
         * the measured rate contributes for half of the next value of
         * the estimated peak rate.
         *
         * So, the first step is to compute the weight as a function
         * of how sequential the workload is. Note that the weight
         * cannot reach 9, because bfqd->sequential_samples cannot
         * become equal to bfqd->peak_rate_samples, which, in its
         * turn, holds true because bfqd->sequential_samples is not
         * incremented for the first sample.
         */
        weight = (9 * bfqd->sequential_samples) / bfqd->peak_rate_samples;

        /*
         * Second step: further refine the weight as a function of the
         * duration of the observation interval.
         */
        weight = min_t(u32, 8,
                       div_u64(weight * bfqd->delta_from_first,
                               BFQ_RATE_REF_INTERVAL));

        /*
         * Divisor ranging from 10, for minimum weight, to 2, for
         * maximum weight.
         */
        divisor = 10 - weight;

        /*
         * Finally, update peak rate:
         *
         * peak_rate = peak_rate * (divisor-1) / divisor  +  rate / divisor
         */
        bfqd->peak_rate *= divisor-1;
        bfqd->peak_rate /= divisor;
        rate /= divisor; /* smoothing constant alpha = 1/divisor */

        bfqd->peak_rate += rate;

        /*
         * For a very slow device, bfqd->peak_rate can reach 0 (see
         * the minimum representable values reported in the comments
         * on BFQ_RATE_SHIFT). Push to 1 if this happens, to avoid
         * divisions by zero where bfqd->peak_rate is used as a
         * divisor.
         */
        bfqd->peak_rate = max_t(u32, 1, bfqd->peak_rate);

        update_thr_responsiveness_params(bfqd);

reset_computation:
        bfq_reset_rate_computation(bfqd, rq);
}

/*
 * Update the read/write peak rate (the main quantity used for
 * auto-tuning, see update_thr_responsiveness_params()).
 *
 * It is not trivial to estimate the peak rate (correctly): because of
 * the presence of sw and hw queues between the scheduler and the
 * device components that finally serve I/O requests, it is hard to
 * say exactly when a given dispatched request is served inside the
 * device, and for how long. As a consequence, it is hard to know
 * precisely at what rate a given set of requests is actually served
 * by the device.
 *
 * On the opposite end, the dispatch time of any request is trivially
 * available, and, from this piece of information, the "dispatch rate"
 * of requests can be immediately computed. So, the idea in the next
 * function is to use what is known, namely request dispatch times
 * (plus, when useful, request completion times), to estimate what is
 * unknown, namely in-device request service rate.
 *
 * The main issue is that, because of the above facts, the rate at
 * which a certain set of requests is dispatched over a certain time
 * interval can vary greatly with respect to the rate at which the
 * same requests are then served. But, since the size of any
 * intermediate queue is limited, and the service scheme is lossless
 * (no request is silently dropped), the following obvious convergence
 * property holds: the number of requests dispatched MUST become
 * closer and closer to the number of requests completed as the
 * observation interval grows. This is the key property used in
 * the next function to estimate the peak service rate as a function
 * of the observed dispatch rate. The function assumes to be invoked
 * on every request dispatch.
 */
static void bfq_update_peak_rate(struct bfq_data *bfqd, struct request *rq)
{
        u64 now_ns = ktime_get_ns();

        if (bfqd->peak_rate_samples == 0) { /* first dispatch */
                bfq_log(bfqd, "update_peak_rate: goto reset, samples %d",
                        bfqd->peak_rate_samples);
                bfq_reset_rate_computation(bfqd, rq);
                goto update_last_values; /* will add one sample */
        }

        /*
         * Device idle for very long: the observation interval lasting
         * up to this dispatch cannot be a valid observation interval
         * for computing a new peak rate (similarly to the late-
         * completion event in bfq_completed_request()). Go to
         * update_rate_and_reset to have the following three steps
         * taken:
         * - close the observation interval at the last (previous)
         *   request dispatch or completion
         * - compute rate, if possible, for that observation interval
         * - start a new observation interval with this dispatch
         */
        if (now_ns - bfqd->last_dispatch > 100*NSEC_PER_MSEC &&
            bfqd->rq_in_driver == 0)
                goto update_rate_and_reset;

        /* Update sampling information */
        bfqd->peak_rate_samples++;

        if ((bfqd->rq_in_driver > 0 ||
                now_ns - bfqd->last_completion < BFQ_MIN_TT)
            && !BFQ_RQ_SEEKY(bfqd, bfqd->last_position, rq))
                bfqd->sequential_samples++;

        bfqd->tot_sectors_dispatched += blk_rq_sectors(rq);

        /* Reset max observed rq size every 32 dispatches */
        if (likely(bfqd->peak_rate_samples % 32))
                bfqd->last_rq_max_size =
                        max_t(u32, blk_rq_sectors(rq), bfqd->last_rq_max_size);
        else
                bfqd->last_rq_max_size = blk_rq_sectors(rq);

        bfqd->delta_from_first = now_ns - bfqd->first_dispatch;

        /* Target observation interval not yet reached, go on sampling */
        if (bfqd->delta_from_first < BFQ_RATE_REF_INTERVAL)
                goto update_last_values;

update_rate_and_reset:
        bfq_update_rate_reset(bfqd, rq);
update_last_values:
        bfqd->last_position = blk_rq_pos(rq) + blk_rq_sectors(rq);
        if (RQ_BFQQ(rq) == bfqd->in_service_queue)
                bfqd->in_serv_last_pos = bfqd->last_position;
        bfqd->last_dispatch = now_ns;
}

/*
 * Remove request from internal lists.
 */
static void bfq_dispatch_remove(struct request_queue *q, struct request *rq)
{
        struct bfq_queue *bfqq = RQ_BFQQ(rq);

        /*
         * For consistency, the next instruction should have been
         * executed after removing the request from the queue and
         * dispatching it.  We execute instead this instruction before
         * bfq_remove_request() (and hence introduce a temporary
         * inconsistency), for efficiency.  In fact, should this
         * dispatch occur for a non in-service bfqq, this anticipated
         * increment prevents two counters related to bfqq->dispatched
         * from risking to be, first, uselessly decremented, and then
         * incremented again when the (new) value of bfqq->dispatched
         * happens to be taken into account.
         */
        bfqq->dispatched++;
        bfq_update_peak_rate(q->elevator->elevator_data, rq);

        bfq_remove_request(q, rq);
}

/*
 * There is a case where idling does not have to be performed for
 * throughput concerns, but to preserve the throughput share of
 * the process associated with bfqq.
 *
 * To introduce this case, we can note that allowing the drive
 * to enqueue more than one request at a time, and hence
 * delegating de facto final scheduling decisions to the
 * drive's internal scheduler, entails loss of control on the
 * actual request service order. In particular, the critical
 * situation is when requests from different processes happen
 * to be present, at the same time, in the internal queue(s)
 * of the drive. In such a situation, the drive, by deciding
 * the service order of the internally-queued requests, does
 * determine also the actual throughput distribution among
 * these processes. But the drive typically has no notion or
 * concern about per-process throughput distribution, and
 * makes its decisions only on a per-request basis. Therefore,
 * the service distribution enforced by the drive's internal
 * scheduler is likely to coincide with the desired throughput
 * distribution only in a completely symmetric, or favorably
 * skewed scenario where:
 * (i-a) each of these processes must get the same throughput as
 *       the others,
 * (i-b) in case (i-a) does not hold, it holds that the process
 *       associated with bfqq must receive a lower or equal
 *       throughput than any of the other processes;
 * (ii)  the I/O of each process has the same properties, in
 *       terms of locality (sequential or random), direction
 *       (reads or writes), request sizes, greediness
 *       (from I/O-bound to sporadic), and so on;

 * In fact, in such a scenario, the drive tends to treat the requests
 * of each process in about the same way as the requests of the
 * others, and thus to provide each of these processes with about the
 * same throughput.  This is exactly the desired throughput
 * distribution if (i-a) holds, or, if (i-b) holds instead, this is an
 * even more convenient distribution for (the process associated with)
 * bfqq.
 *
 * In contrast, in any asymmetric or unfavorable scenario, device
 * idling (I/O-dispatch plugging) is certainly needed to guarantee
 * that bfqq receives its assigned fraction of the device throughput
 * (see [1] for details).
 *
 * The problem is that idling may significantly reduce throughput with
 * certain combinations of types of I/O and devices. An important
 * example is sync random I/O on flash storage with command
 * queueing. So, unless bfqq falls in cases where idling also boosts
 * throughput, it is important to check conditions (i-a), i(-b) and
 * (ii) accurately, so as to avoid idling when not strictly needed for
 * service guarantees.
 *
 * Unfortunately, it is extremely difficult to thoroughly check
 * condition (ii). And, in case there are active groups, it becomes
 * very difficult to check conditions (i-a) and (i-b) too.  In fact,
 * if there are active groups, then, for conditions (i-a) or (i-b) to
 * become false 'indirectly', it is enough that an active group
 * contains more active processes or sub-groups than some other active
 * group. More precisely, for conditions (i-a) or (i-b) to become
 * false because of such a group, it is not even necessary that the
 * group is (still) active: it is sufficient that, even if the group
 * has become inactive, some of its descendant processes still have
 * some request already dispatched but still waiting for
 * completion. In fact, requests have still to be guaranteed their
 * share of the throughput even after being dispatched. In this
 * respect, it is easy to show that, if a group frequently becomes
 * inactive while still having in-flight requests, and if, when this
 * happens, the group is not considered in the calculation of whether
 * the scenario is asymmetric, then the group may fail to be
 * guaranteed its fair share of the throughput (basically because
 * idling may not be performed for the descendant processes of the
 * group, but it had to be).  We address this issue with the following
 * bi-modal behavior, implemented in the function
 * bfq_asymmetric_scenario().
 *
 * If there are groups with requests waiting for completion
 * (as commented above, some of these groups may even be
 * already inactive), then the scenario is tagged as
 * asymmetric, conservatively, without checking any of the
 * conditions (i-a), (i-b) or (ii). So the device is idled for bfqq.
 * This behavior matches also the fact that groups are created
 * exactly if controlling I/O is a primary concern (to
 * preserve bandwidth and latency guarantees).
 *
 * On the opposite end, if there are no groups with requests waiting
 * for completion, then only conditions (i-a) and (i-b) are actually
 * controlled, i.e., provided that conditions (i-a) or (i-b) holds,
 * idling is not performed, regardless of whether condition (ii)
 * holds.  In other words, only if conditions (i-a) and (i-b) do not
 * hold, then idling is allowed, and the device tends to be prevented
 * from queueing many requests, possibly of several processes. Since
 * there are no groups with requests waiting for completion, then, to
 * control conditions (i-a) and (i-b) it is enough to check just
 * whether all the queues with requests waiting for completion also
 * have the same weight.
 *
 * Not checking condition (ii) evidently exposes bfqq to the
 * risk of getting less throughput than its fair share.
 * However, for queues with the same weight, a further
 * mechanism, preemption, mitigates or even eliminates this
 * problem. And it does so without consequences on overall
 * throughput. This mechanism and its benefits are explained
 * in the next three paragraphs.
 *
 * Even if a queue, say Q, is expired when it remains idle, Q
 * can still preempt the new in-service queue if the next
 * request of Q arrives soon (see the comments on
 * bfq_bfqq_update_budg_for_activation). If all queues and
 * groups have the same weight, this form of preemption,
 * combined with the hole-recovery heuristic described in the
 * comments on function bfq_bfqq_update_budg_for_activation,
 * are enough to preserve a correct bandwidth distribution in
 * the mid term, even without idling. In fact, even if not
 * idling allows the internal queues of the device to contain
 * many requests, and thus to reorder requests, we can rather
 * safely assume that the internal scheduler still preserves a
 * minimum of mid-term fairness.
 *
 * More precisely, this preemption-based, idleless approach
 * provides fairness in terms of IOPS, and not sectors per
 * second. This can be seen with a simple example. Suppose
 * that there are two queues with the same weight, but that
 * the first queue receives requests of 8 sectors, while the
 * second queue receives requests of 1024 sectors. In
 * addition, suppose that each of the two queues contains at
 * most one request at a time, which implies that each queue
 * always remains idle after it is served. Finally, after
 * remaining idle, each queue receives very quickly a new
 * request. It follows that the two queues are served
 * alternatively, preempting each other if needed. This
 * implies that, although both queues have the same weight,
 * the queue with large requests receives a service that is
 * 1024/8 times as high as the service received by the other
 * queue.
 *
 * The motivation for using preemption instead of idling (for
 * queues with the same weight) is that, by not idling,
 * service guarantees are preserved (completely or at least in
 * part) without minimally sacrificing throughput. And, if
 * there is no active group, then the primary expectation for
 * this device is probably a high throughput.
 *
 * We are now left only with explaining the two sub-conditions in the
 * additional compound condition that is checked below for deciding
 * whether the scenario is asymmetric. To explain the first
 * sub-condition, we need to add that the function
 * bfq_asymmetric_scenario checks the weights of only
 * non-weight-raised queues, for efficiency reasons (see comments on
 * bfq_weights_tree_add()). Then the fact that bfqq is weight-raised
 * is checked explicitly here. More precisely, the compound condition
 * below takes into account also the fact that, even if bfqq is being
 * weight-raised, the scenario is still symmetric if all queues with
 * requests waiting for completion happen to be
 * weight-raised. Actually, we should be even more precise here, and
 * differentiate between interactive weight raising and soft real-time
 * weight raising.
 *
 * The second sub-condition checked in the compound condition is
 * whether there is a fair amount of already in-flight I/O not
 * belonging to bfqq. If so, I/O dispatching is to be plugged, for the
 * following reason. The drive may decide to serve in-flight
 * non-bfqq's I/O requests before bfqq's ones, thereby delaying the
 * arrival of new I/O requests for bfqq (recall that bfqq is sync). If
 * I/O-dispatching is not plugged, then, while bfqq remains empty, a
 * basically uncontrolled amount of I/O from other queues may be
 * dispatched too, possibly causing the service of bfqq's I/O to be
 * delayed even longer in the drive. This problem gets more and more
 * serious as the speed and the queue depth of the drive grow,
 * because, as these two quantities grow, the probability to find no
 * queue busy but many requests in flight grows too. By contrast,
 * plugging I/O dispatching minimizes the delay induced by already
 * in-flight I/O, and enables bfqq to recover the bandwidth it may
 * lose because of this delay.
 *
 * As a side note, it is worth considering that the above
 * device-idling countermeasures may however fail in the following
 * unlucky scenario: if I/O-dispatch plugging is (correctly) disabled
 * in a time period during which all symmetry sub-conditions hold, and
 * therefore the device is allowed to enqueue many requests, but at
 * some later point in time some sub-condition stops to hold, then it
 * may become impossible to make requests be served in the desired
 * order until all the requests already queued in the device have been
 * served. The last sub-condition commented above somewhat mitigates
 * this problem for weight-raised queues.
 */
static bool idling_needed_for_service_guarantees(struct bfq_data *bfqd,
                                                 struct bfq_queue *bfqq)
{
        /* No point in idling for bfqq if it won't get requests any longer */
        if (unlikely(!bfqq_process_refs(bfqq)))
                return false;

        return (bfqq->wr_coeff > 1 &&
                (bfqd->wr_busy_queues <
                 bfq_tot_busy_queues(bfqd) ||
                 bfqd->rq_in_driver >=
                 bfqq->dispatched + 4)) ||
                bfq_asymmetric_scenario(bfqd, bfqq);
}

static bool __bfq_bfqq_expire(struct bfq_data *bfqd, struct bfq_queue *bfqq,
                              enum bfqq_expiration reason)
{
        /*
         * If this bfqq is shared between multiple processes, check
         * to make sure that those processes are still issuing I/Os
         * within the mean seek distance. If not, it may be time to
         * break the queues apart again.
         */
        if (bfq_bfqq_coop(bfqq) && BFQQ_SEEKY(bfqq))
                bfq_mark_bfqq_split_coop(bfqq);

        /*
         * Consider queues with a higher finish virtual time than
         * bfqq. If idling_needed_for_service_guarantees(bfqq) returns
         * true, then bfqq's bandwidth would be violated if an
         * uncontrolled amount of I/O from these queues were
         * dispatched while bfqq is waiting for its new I/O to
         * arrive. This is exactly what may happen if this is a forced
         * expiration caused by a preemption attempt, and if bfqq is
         * not re-scheduled. To prevent this from happening, re-queue
         * bfqq if it needs I/O-dispatch plugging, even if it is
         * empty. By doing so, bfqq is granted to be served before the
         * above queues (provided that bfqq is of course eligible).
         */
        if (RB_EMPTY_ROOT(&bfqq->sort_list) &&
            !(reason == BFQQE_PREEMPTED &&
              idling_needed_for_service_guarantees(bfqd, bfqq))) {
                if (bfqq->dispatched == 0)
                        /*
                         * Overloading budget_timeout field to store
                         * the time at which the queue remains with no
                         * backlog and no outstanding request; used by
                         * the weight-raising mechanism.
                         */
                        bfqq->budget_timeout = jiffies;

                bfq_del_bfqq_busy(bfqd, bfqq, true);
        } else {
                bfq_requeue_bfqq(bfqd, bfqq, true);
                /*
                 * Resort priority tree of potential close cooperators.
                 * See comments on bfq_pos_tree_add_move() for the unlikely().
                 */
                if (unlikely(!bfqd->nonrot_with_queueing &&
                             !RB_EMPTY_ROOT(&bfqq->sort_list)))
                        bfq_pos_tree_add_move(bfqd, bfqq);
        }

        /*
         * All in-service entities must have been properly deactivated
         * or requeued before executing the next function, which
         * resets all in-service entities as no more in service. This
         * may cause bfqq to be freed. If this happens, the next
         * function returns true.
         */
        return __bfq_bfqd_reset_in_service(bfqd);
}

/**
 * __bfq_bfqq_recalc_budget - try to adapt the budget to the @bfqq behavior.
 * @bfqd: device data.
 * @bfqq: queue to update.
 * @reason: reason for expiration.
 *
 * Handle the feedback on @bfqq budget at queue expiration.
 * See the body for detailed comments.
 */
static void __bfq_bfqq_recalc_budget(struct bfq_data *bfqd,
                                     struct bfq_queue *bfqq,
                                     enum bfqq_expiration reason)
{
        struct request *next_rq;
        int budget, min_budget;

        min_budget = bfq_min_budget(bfqd);

        if (bfqq->wr_coeff == 1)
                budget = bfqq->max_budget;
        else /*
              * Use a constant, low budget for weight-raised queues,
              * to help achieve a low latency. Keep it slightly higher
              * than the minimum possible budget, to cause a little
              * bit fewer expirations.
              */
                budget = 2 * min_budget;

        bfq_log_bfqq(bfqd, bfqq, "recalc_budg: last budg %d, budg left %d",
                bfqq->entity.budget, bfq_bfqq_budget_left(bfqq));
        bfq_log_bfqq(bfqd, bfqq, "recalc_budg: last max_budg %d, min budg %d",
                budget, bfq_min_budget(bfqd));
        bfq_log_bfqq(bfqd, bfqq, "recalc_budg: sync %d, seeky %d",
                bfq_bfqq_sync(bfqq), BFQQ_SEEKY(bfqd->in_service_queue));

        if (bfq_bfqq_sync(bfqq) && bfqq->wr_coeff == 1) {
                switch (reason) {
                /*
                 * Caveat: in all the following cases we trade latency
                 * for throughput.
                 */
                case BFQQE_TOO_IDLE:
                        /*
                         * This is the only case where we may reduce
                         * the budget: if there is no request of the
                         * process still waiting for completion, then
                         * we assume (tentatively) that the timer has
                         * expired because the batch of requests of
                         * the process could have been served with a
                         * smaller budget.  Hence, betting that
                         * process will behave in the same way when it
                         * becomes backlogged again, we reduce its
                         * next budget.  As long as we guess right,
                         * this budget cut reduces the latency
                         * experienced by the process.
                         *
                         * However, if there are still outstanding
                         * requests, then the process may have not yet
                         * issued its next request just because it is
                         * still waiting for the completion of some of
                         * the still outstanding ones.  So in this
                         * subcase we do not reduce its budget, on the
                         * contrary we increase it to possibly boost
                         * the throughput, as discussed in the
                         * comments to the BUDGET_TIMEOUT case.
                         */
                        if (bfqq->dispatched > 0) /* still outstanding reqs */
                                budget = min(budget * 2, bfqd->bfq_max_budget);
                        else {
                                if (budget > 5 * min_budget)
                                        budget -= 4 * min_budget;
                                else
                                        budget = min_budget;
                        }
                        break;
                case BFQQE_BUDGET_TIMEOUT:
                        /*
                         * We double the budget here because it gives
                         * the chance to boost the throughput if this
                         * is not a seeky process (and has bumped into
                         * this timeout because of, e.g., ZBR).
                         */
                        budget = min(budget * 2, bfqd->bfq_max_budget);
                        break;
                case BFQQE_BUDGET_EXHAUSTED:
                        /*
                         * The process still has backlog, and did not
                         * let either the budget timeout or the disk
                         * idling timeout expire. Hence it is not
                         * seeky, has a short thinktime and may be
                         * happy with a higher budget too. So
                         * definitely increase the budget of this good
                         * candidate to boost the disk throughput.
                         */
                        budget = min(budget * 4, bfqd->bfq_max_budget);
                        break;
                case BFQQE_NO_MORE_REQUESTS:
                        /*
                         * For queues that expire for this reason, it
                         * is particularly important to keep the
                         * budget close to the actual service they
                         * need. Doing so reduces the timestamp
                         * misalignment problem described in the
                         * comments in the body of
                         * __bfq_activate_entity. In fact, suppose
                         * that a queue systematically expires for
                         * BFQQE_NO_MORE_REQUESTS and presents a
                         * new request in time to enjoy timestamp
                         * back-shifting. The larger the budget of the
                         * queue is with respect to the service the
                         * queue actually requests in each service
                         * slot, the more times the queue can be
                         * reactivated with the same virtual finish
                         * time. It follows that, even if this finish
                         * time is pushed to the system virtual time
                         * to reduce the consequent timestamp
                         * misalignment, the queue unjustly enjoys for
                         * many re-activations a lower finish time
                         * than all newly activated queues.
                         *
                         * The service needed by bfqq is measured
                         * quite precisely by bfqq->entity.service.
                         * Since bfqq does not enjoy device idling,
                         * bfqq->entity.service is equal to the number
                         * of sectors that the process associated with
                         * bfqq requested to read/write before waiting
                         * for request completions, or blocking for
                         * other reasons.
                         */
                        budget = max_t(int, bfqq->entity.service, min_budget);
                        break;
                default:
                        return;
                }
        } else if (!bfq_bfqq_sync(bfqq)) {
                /*
                 * Async queues get always the maximum possible
                 * budget, as for them we do not care about latency
                 * (in addition, their ability to dispatch is limited
                 * by the charging factor).
                 */
                budget = bfqd->bfq_max_budget;
        }

        bfqq->max_budget = budget;

        if (bfqd->budgets_assigned >= bfq_stats_min_budgets &&
            !bfqd->bfq_user_max_budget)
                bfqq->max_budget = min(bfqq->max_budget, bfqd->bfq_max_budget);

        /*
         * If there is still backlog, then assign a new budget, making
         * sure that it is large enough for the next request.  Since
         * the finish time of bfqq must be kept in sync with the
         * budget, be sure to call __bfq_bfqq_expire() *after* this
         * update.
         *
         * If there is no backlog, then no need to update the budget;
         * it will be updated on the arrival of a new request.
         */
        next_rq = bfqq->next_rq;
        if (next_rq)
                bfqq->entity.budget = max_t(unsigned long, bfqq->max_budget,
                                            bfq_serv_to_charge(next_rq, bfqq));

        bfq_log_bfqq(bfqd, bfqq, "head sect: %u, new budget %d",
                        next_rq ? blk_rq_sectors(next_rq) : 0,
                        bfqq->entity.budget);
}

/*
 * Return true if the process associated with bfqq is "slow". The slow
 * flag is used, in addition to the budget timeout, to reduce the
 * amount of service provided to seeky processes, and thus reduce
 * their chances to lower the throughput. More details in the comments
 * on the function bfq_bfqq_expire().
 *
 * An important observation is in order: as discussed in the comments
 * on the function bfq_update_peak_rate(), with devices with internal
 * queues, it is hard if ever possible to know when and for how long
 * an I/O request is processed by the device (apart from the trivial
 * I/O pattern where a new request is dispatched only after the
 * previous one has been completed). This makes it hard to evaluate
 * the real rate at which the I/O requests of each bfq_queue are
 * served.  In fact, for an I/O scheduler like BFQ, serving a
 * bfq_queue means just dispatching its requests during its service
 * slot (i.e., until the budget of the queue is exhausted, or the
 * queue remains idle, or, finally, a timeout fires). But, during the
 * service slot of a bfq_queue, around 100 ms at most, the device may
 * be even still processing requests of bfq_queues served in previous
 * service slots. On the opposite end, the requests of the in-service
 * bfq_queue may be completed after the service slot of the queue
 * finishes.
 *
 * Anyway, unless more sophisticated solutions are used
 * (where possible), the sum of the sizes of the requests dispatched
 * during the service slot of a bfq_queue is probably the only
 * approximation available for the service received by the bfq_queue
 * during its service slot. And this sum is the quantity used in this
 * function to evaluate the I/O speed of a process.
 */
static bool bfq_bfqq_is_slow(struct bfq_data *bfqd, struct bfq_queue *bfqq,
                                 bool compensate, enum bfqq_expiration reason,
                                 unsigned long *delta_ms)
{
        ktime_t delta_ktime;
        u32 delta_usecs;
        bool slow = BFQQ_SEEKY(bfqq); /* if delta too short, use seekyness */

        if (!bfq_bfqq_sync(bfqq))
                return false;

        if (compensate)
                delta_ktime = bfqd->last_idling_start;
        else
                delta_ktime = ktime_get();
        delta_ktime = ktime_sub(delta_ktime, bfqd->last_budget_start);
        delta_usecs = ktime_to_us(delta_ktime);

        /* don't use too short time intervals */
        if (delta_usecs < 1000) {
                if (blk_queue_nonrot(bfqd->queue))
                         /*
                          * give same worst-case guarantees as idling
                          * for seeky
                          */
                        *delta_ms = BFQ_MIN_TT / NSEC_PER_MSEC;
                else /* charge at least one seek */
                        *delta_ms = bfq_slice_idle / NSEC_PER_MSEC;

                return slow;
        }

        *delta_ms = delta_usecs / USEC_PER_MSEC;

        /*
         * Use only long (> 20ms) intervals to filter out excessive
         * spikes in service rate estimation.
         */
        if (delta_usecs > 20000) {
                /*
                 * Caveat for rotational devices: processes doing I/O
                 * in the slower disk zones tend to be slow(er) even
                 * if not seeky. In this respect, the estimated peak
                 * rate is likely to be an average over the disk
                 * surface. Accordingly, to not be too harsh with
                 * unlucky processes, a process is deemed slow only if
                 * its rate has been lower than half of the estimated
                 * peak rate.
                 */
                slow = bfqq->entity.service < bfqd->bfq_max_budget / 2;
        }

        bfq_log_bfqq(bfqd, bfqq, "bfq_bfqq_is_slow: slow %d", slow);

        return slow;
}

/*
 * To be deemed as soft real-time, an application must meet two
 * requirements. First, the application must not require an average
 * bandwidth higher than the approximate bandwidth required to playback or
 * record a compressed high-definition video.
 * The next function is invoked on the completion of the last request of a
 * batch, to compute the next-start time instant, soft_rt_next_start, such
 * that, if the next request of the application does not arrive before
 * soft_rt_next_start, then the above requirement on the bandwidth is met.
 *
 * The second requirement is that the request pattern of the application is
 * isochronous, i.e., that, after issuing a request or a batch of requests,
 * the application stops issuing new requests until all its pending requests
 * have been completed. After that, the application may issue a new batch,
 * and so on.
 * For this reason the next function is invoked to compute
 * soft_rt_next_start only for applications that meet this requirement,
 * whereas soft_rt_next_start is set to infinity for applications that do
 * not.
 *
 * Unfortunately, even a greedy (i.e., I/O-bound) application may
 * happen to meet, occasionally or systematically, both the above
 * bandwidth and isochrony requirements. This may happen at least in
 * the following circumstances. First, if the CPU load is high. The
 * application may stop issuing requests while the CPUs are busy
 * serving other processes, then restart, then stop again for a while,
 * and so on. The other circumstances are related to the storage
 * device: the storage device is highly loaded or reaches a low-enough
 * throughput with the I/O of the application (e.g., because the I/O
 * is random and/or the device is slow). In all these cases, the
 * I/O of the application may be simply slowed down enough to meet
 * the bandwidth and isochrony requirements. To reduce the probability
 * that greedy applications are deemed as soft real-time in these
 * corner cases, a further rule is used in the computation of
 * soft_rt_next_start: the return value of this function is forced to
 * be higher than the maximum between the following two quantities.
 *
 * (a) Current time plus: (1) the maximum time for which the arrival
 *     of a request is waited for when a sync queue becomes idle,
 *     namely bfqd->bfq_slice_idle, and (2) a few extra jiffies. We
 *     postpone for a moment the reason for adding a few extra
 *     jiffies; we get back to it after next item (b).  Lower-bounding
 *     the return value of this function with the current time plus
 *     bfqd->bfq_slice_idle tends to filter out greedy applications,
 *     because the latter issue their next request as soon as possible
 *     after the last one has been completed. In contrast, a soft
 *     real-time application spends some time processing data, after a
 *     batch of its requests has been completed.
 *
 * (b) Current value of bfqq->soft_rt_next_start. As pointed out
 *     above, greedy applications may happen to meet both the
 *     bandwidth and isochrony requirements under heavy CPU or
 *     storage-device load. In more detail, in these scenarios, these
 *     applications happen, only for limited time periods, to do I/O
 *     slowly enough to meet all the requirements described so far,
 *     including the filtering in above item (a). These slow-speed
 *     time intervals are usually interspersed between other time
 *     intervals during which these applications do I/O at a very high
 *     speed. Fortunately, exactly because of the high speed of the
 *     I/O in the high-speed intervals, the values returned by this
 *     function happen to be so high, near the end of any such
 *     high-speed interval, to be likely to fall *after* the end of
 *     the low-speed time interval that follows. These high values are
 *     stored in bfqq->soft_rt_next_start after each invocation of
 *     this function. As a consequence, if the last value of
 *     bfqq->soft_rt_next_start is constantly used to lower-bound the
 *     next value that this function may return, then, from the very
 *     beginning of a low-speed interval, bfqq->soft_rt_next_start is
 *     likely to be constantly kept so high that any I/O request
 *     issued during the low-speed interval is considered as arriving
 *     to soon for the application to be deemed as soft
 *     real-time. Then, in the high-speed interval that follows, the
 *     application will not be deemed as soft real-time, just because
 *     it will do I/O at a high speed. And so on.
 *
 * Getting back to the filtering in item (a), in the following two
 * cases this filtering might be easily passed by a greedy
 * application, if the reference quantity was just
 * bfqd->bfq_slice_idle:
 * 1) HZ is so low that the duration of a jiffy is comparable to or
 *    higher than bfqd->bfq_slice_idle. This happens, e.g., on slow
 *    devices with HZ=100. The time granularity may be so coarse
 *    that the approximation, in jiffies, of bfqd->bfq_slice_idle
 *    is rather lower than the exact value.
 * 2) jiffies, instead of increasing at a constant rate, may stop increasing
 *    for a while, then suddenly 'jump' by several units to recover the lost
 *    increments. This seems to happen, e.g., inside virtual machines.
 * To address this issue, in the filtering in (a) we do not use as a
 * reference time interval just bfqd->bfq_slice_idle, but
 * bfqd->bfq_slice_idle plus a few jiffies. In particular, we add the
 * minimum number of jiffies for which the filter seems to be quite
 * precise also in embedded systems and KVM/QEMU virtual machines.
 */
static unsigned long bfq_bfqq_softrt_next_start(struct bfq_data *bfqd,
                                                struct bfq_queue *bfqq)
{
        return max3(bfqq->soft_rt_next_start,
                    bfqq->last_idle_bklogged +
                    HZ * bfqq->service_from_backlogged /
                    bfqd->bfq_wr_max_softrt_rate,
                    jiffies + nsecs_to_jiffies(bfqq->bfqd->bfq_slice_idle) + 4);
}

/**
 * bfq_bfqq_expire - expire a queue.
 * @bfqd: device owning the queue.
 * @bfqq: the queue to expire.
 * @compensate: if true, compensate for the time spent idling.
 * @reason: the reason causing the expiration.
 *
 * If the process associated with bfqq does slow I/O (e.g., because it
 * issues random requests), we charge bfqq with the time it has been
 * in service instead of the service it has received (see
 * bfq_bfqq_charge_time for details on how this goal is achieved). As
 * a consequence, bfqq will typically get higher timestamps upon
 * reactivation, and hence it will be rescheduled as if it had
 * received more service than what it has actually received. In the
 * end, bfqq receives less service in proportion to how slowly its
 * associated process consumes its budgets (and hence how seriously it
 * tends to lower the throughput). In addition, this time-charging
 * strategy guarantees time fairness among slow processes. In
 * contrast, if the process associated with bfqq is not slow, we
 * charge bfqq exactly with the service it has received.
 *
 * Charging time to the first type of queues and the exact service to
 * the other has the effect of using the WF2Q+ policy to schedule the
 * former on a timeslice basis, without violating service domain
 * guarantees among the latter.
 */
void bfq_bfqq_expire(struct bfq_data *bfqd,
                     struct bfq_queue *bfqq,
                     bool compensate,
                     enum bfqq_expiration reason)
{
        bool slow;
        unsigned long delta = 0;
        struct bfq_entity *entity = &bfqq->entity;

        /*
         * Check whether the process is slow (see bfq_bfqq_is_slow).
         */
        slow = bfq_bfqq_is_slow(bfqd, bfqq, compensate, reason, &delta);

        /*
         * As above explained, charge slow (typically seeky) and
         * timed-out queues with the time and not the service
         * received, to favor sequential workloads.
         *
         * Processes doing I/O in the slower disk zones will tend to
         * be slow(er) even if not seeky. Therefore, since the
         * estimated peak rate is actually an average over the disk
         * surface, these processes may timeout just for bad luck. To
         * avoid punishing them, do not charge time to processes that
         * succeeded in consuming at least 2/3 of their budget. This
         * allows BFQ to preserve enough elasticity to still perform
         * bandwidth, and not time, distribution with little unlucky
         * or quasi-sequential processes.
         */
        if (bfqq->wr_coeff == 1 &&
            (slow ||
             (reason == BFQQE_BUDGET_TIMEOUT &&
              bfq_bfqq_budget_left(bfqq) >=  entity->budget / 3)))
                bfq_bfqq_charge_time(bfqd, bfqq, delta);

        if (reason == BFQQE_TOO_IDLE &&
            entity->service <= 2 * entity->budget / 10)
                bfq_clear_bfqq_IO_bound(bfqq);

        if (bfqd->low_latency && bfqq->wr_coeff == 1)
                bfqq->last_wr_start_finish = jiffies;

        if (bfqd->low_latency && bfqd->bfq_wr_max_softrt_rate > 0 &&
            RB_EMPTY_ROOT(&bfqq->sort_list)) {
                /*
                 * If we get here, and there are no outstanding
                 * requests, then the request pattern is isochronous
                 * (see the comments on the function
                 * bfq_bfqq_softrt_next_start()). Thus we can compute
                 * soft_rt_next_start. And we do it, unless bfqq is in
                 * interactive weight raising. We do not do it in the
                 * latter subcase, for the following reason. bfqq may
                 * be conveying the I/O needed to load a soft
                 * real-time application. Such an application will
                 * actually exhibit a soft real-time I/O pattern after
                 * it finally starts doing its job. But, if
                 * soft_rt_next_start is computed here for an
                 * interactive bfqq, and bfqq had received a lot of
                 * service before remaining with no outstanding
                 * request (likely to happen on a fast device), then
                 * soft_rt_next_start would be assigned such a high
                 * value that, for a very long time, bfqq would be
                 * prevented from being possibly considered as soft
                 * real time.
                 *
                 * If, instead, the queue still has outstanding
                 * requests, then we have to wait for the completion
                 * of all the outstanding requests to discover whether
                 * the request pattern is actually isochronous.
                 */
                if (bfqq->dispatched == 0 &&
                    bfqq->wr_coeff != bfqd->bfq_wr_coeff)
                        bfqq->soft_rt_next_start =
                                bfq_bfqq_softrt_next_start(bfqd, bfqq);
                else if (bfqq->dispatched > 0) {
                        /*
                         * Schedule an update of soft_rt_next_start to when
                         * the task may be discovered to be isochronous.
                         */
                        bfq_mark_bfqq_softrt_update(bfqq);
                }
        }

        bfq_log_bfqq(bfqd, bfqq,
                "expire (%d, slow %d, num_disp %d, short_ttime %d)", reason,
                slow, bfqq->dispatched, bfq_bfqq_has_short_ttime(bfqq));

        /*
         * bfqq expired, so no total service time needs to be computed
         * any longer: reset state machine for measuring total service
         * times.
         */
        bfqd->rqs_injected = bfqd->wait_dispatch = false;
        bfqd->waited_rq = NULL;