/* Copyright (C) 2013 Cisco Systems, Inc, 2013.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * Author: Vijay Subramanian <vijaynsu@cisco.com>
 * Author: Mythili Prabhu <mysuryan@cisco.com>
 *
 * ECN support is added by Naeem Khademi <naeemk@ifi.uio.no>
 * University of Oslo, Norway.
 *
 * References:
 * IETF draft submission: http://tools.ietf.org/html/draft-pan-aqm-pie-00
 * IEEE  Conference on High Performance Switching and Routing 2013 :
 * "PIE: A * Lightweight Control Scheme to Address the Bufferbloat Problem"
 */

#include <linux/module.h>
#include <linux/slab.h>
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/skbuff.h>
#include <net/pkt_sched.h>
#include <net/inet_ecn.h>

#define QUEUE_THRESHOLD 10000
#define DQCOUNT_INVALID -1
#define MAX_PROB  0xffffffff
#define PIE_SCALE 8

/* parameters used */
struct pie_params {
        psched_time_t target;   /* user specified target delay in pschedtime */
        u32 tupdate;            /* timer frequency (in jiffies) */
        u32 limit;              /* number of packets that can be enqueued */
        u32 alpha;              /* alpha and beta are between 0 and 32 */
        u32 beta;               /* and are used for shift relative to 1 */
        bool ecn;               /* true if ecn is enabled */
        bool bytemode;          /* to scale drop early prob based on pkt size */
};

/* variables used */
struct pie_vars {
        u32 prob;               /* probability but scaled by u32 limit. */
        psched_time_t burst_time;
        psched_time_t qdelay;
        psched_time_t qdelay_old;
        u64 dq_count;           /* measured in bytes */
        psched_time_t dq_tstamp;        /* drain rate */
        u32 avg_dq_rate;        /* bytes per pschedtime tick,scaled */
        u32 qlen_old;           /* in bytes */
};

/* statistics gathering */
struct pie_stats {
        u32 packets_in;         /* total number of packets enqueued */
        u32 dropped;            /* packets dropped due to pie_action */
        u32 overlimit;          /* dropped due to lack of space in queue */
        u32 maxq;               /* maximum queue size */
        u32 ecn_mark;           /* packets marked with ECN */
};

/* private data for the Qdisc */
struct pie_sched_data {
        struct pie_params params;
        struct pie_vars vars;
        struct pie_stats stats;
        struct timer_list adapt_timer;
        struct Qdisc *sch;
};

static void pie_params_init(struct pie_params *params)
{
        params->alpha = 2;
        params->beta = 20;
        params->tupdate = usecs_to_jiffies(30 * USEC_PER_MSEC); /* 30 ms */
        params->limit = 1000;   /* default of 1000 packets */
        params->target = PSCHED_NS2TICKS(20 * NSEC_PER_MSEC);   /* 20 ms */
        params->ecn = false;
        params->bytemode = false;
}

static void pie_vars_init(struct pie_vars *vars)
{
        vars->dq_count = DQCOUNT_INVALID;
        vars->avg_dq_rate = 0;
        /* default of 100 ms in pschedtime */
        vars->burst_time = PSCHED_NS2TICKS(100 * NSEC_PER_MSEC);
}

static bool drop_early(struct Qdisc *sch, u32 packet_size)
{
        struct pie_sched_data *q = qdisc_priv(sch);
        u32 rnd;
        u32 local_prob = q->vars.prob;
        u32 mtu = psched_mtu(qdisc_dev(sch));

        /* If there is still burst allowance left skip random early drop */
        if (q->vars.burst_time > 0)
                return false;

        /* If current delay is less than half of target, and
         * if drop prob is low already, disable early_drop
         */
        if ((q->vars.qdelay < q->params.target / 2)
            && (q->vars.prob < MAX_PROB / 5))
                return false;

        /* If we have fewer than 2 mtu-sized packets, disable drop_early,
         * similar to min_th in RED
         */
        if (sch->qstats.backlog < 2 * mtu)
                return false;

        /* If bytemode is turned on, use packet size to compute new
         * probablity. Smaller packets will have lower drop prob in this case
         */
        if (q->params.bytemode && packet_size <= mtu)
                local_prob = (local_prob / mtu) * packet_size;
        else
                local_prob = q->vars.prob;

        rnd = prandom_u32();
        if (rnd < local_prob)
                return true;

        return false;
}

static int pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
                             struct sk_buff **to_free)
{
        struct pie_sched_data *q = qdisc_priv(sch);
        bool enqueue = false;

        if (unlikely(qdisc_qlen(sch) >= sch->limit)) {
                q->stats.overlimit++;
                goto out;
        }

        if (!drop_early(sch, skb->len)) {
                enqueue = true;
        } else if (q->params.ecn && (q->vars.prob <= MAX_PROB / 10) &&
                   INET_ECN_set_ce(skb)) {
                /* If packet is ecn capable, mark it if drop probability
                 * is lower than 10%, else drop it.
                 */
                q->stats.ecn_mark++;
                enqueue = true;
        }

        /* we can enqueue the packet */
        if (enqueue) {
                q->stats.packets_in++;
                if (qdisc_qlen(sch) > q->stats.maxq)
                        q->stats.maxq = qdisc_qlen(sch);

                return qdisc_enqueue_tail(skb, sch);
        }

out:
        q->stats.dropped++;
        return qdisc_drop(skb, sch, to_free);
}

static const struct nla_policy pie_policy[TCA_PIE_MAX + 1] = {
        [TCA_PIE_TARGET] = {.type = NLA_U32},
        [TCA_PIE_LIMIT] = {.type = NLA_U32},
        [TCA_PIE_TUPDATE] = {.type = NLA_U32},
        [TCA_PIE_ALPHA] = {.type = NLA_U32},
        [TCA_PIE_BETA] = {.type = NLA_U32},
        [TCA_PIE_ECN] = {.type = NLA_U32},
        [TCA_PIE_BYTEMODE] = {.type = NLA_U32},
};

static int pie_change(struct Qdisc *sch, struct nlattr *opt,
                      struct netlink_ext_ack *extack)
{
        struct pie_sched_data *q = qdisc_priv(sch);
        struct nlattr *tb[TCA_PIE_MAX + 1];
        unsigned int qlen, dropped = 0;
        int err;

        if (!opt)
                return -EINVAL;

        err = nla_parse_nested(tb, TCA_PIE_MAX, opt, pie_policy, NULL);
        if (err < 0)
                return err;

        sch_tree_lock(sch);

        /* convert from microseconds to pschedtime */
        if (tb[TCA_PIE_TARGET]) {
                /* target is in us */
                u32 target = nla_get_u32(tb[TCA_PIE_TARGET]);

                /* convert to pschedtime */
                q->params.target = PSCHED_NS2TICKS((u64)target * NSEC_PER_USEC);
        }

        /* tupdate is in jiffies */
        if (tb[TCA_PIE_TUPDATE])
                q->params.tupdate = usecs_to_jiffies(nla_get_u32(tb[TCA_PIE_TUPDATE]));

        if (tb[TCA_PIE_LIMIT]) {
                u32 limit = nla_get_u32(tb[TCA_PIE_LIMIT]);

                q->params.limit = limit;
                sch->limit = limit;
        }

        if (tb[TCA_PIE_ALPHA])
                q->params.alpha = nla_get_u32(tb[TCA_PIE_ALPHA]);

        if (tb[TCA_PIE_BETA])
                q->params.beta = nla_get_u32(tb[TCA_PIE_BETA]);

        if (tb[TCA_PIE_ECN])
                q->params.ecn = nla_get_u32(tb[TCA_PIE_ECN]);

        if (tb[TCA_PIE_BYTEMODE])
                q->params.bytemode = nla_get_u32(tb[TCA_PIE_BYTEMODE]);

        /* Drop excess packets if new limit is lower */
        qlen = sch->q.qlen;
        while (sch->q.qlen > sch->limit) {
                struct sk_buff *skb = __qdisc_dequeue_head(&sch->q);

                dropped += qdisc_pkt_len(skb);
                qdisc_qstats_backlog_dec(sch, skb);
                rtnl_qdisc_drop(skb, sch);
        }
        qdisc_tree_reduce_backlog(sch, qlen - sch->q.qlen, dropped);

        sch_tree_unlock(sch);
        return 0;
}

static void pie_process_dequeue(struct Qdisc *sch, struct sk_buff *skb)
{

        struct pie_sched_data *q = qdisc_priv(sch);
        int qlen = sch->qstats.backlog; /* current queue size in bytes */

        /* If current queue is about 10 packets or more and dq_count is unset
         * we have enough packets to calculate the drain rate. Save
         * current time as dq_tstamp and start measurement cycle.
         */
        if (qlen >= QUEUE_THRESHOLD && q->vars.dq_count == DQCOUNT_INVALID) {
                q->vars.dq_tstamp = psched_get_time();
                q->vars.dq_count = 0;
        }

        /* Calculate the average drain rate from this value.  If queue length
         * has receded to a small value viz., <= QUEUE_THRESHOLD bytes,reset
         * the dq_count to -1 as we don't have enough packets to calculate the
         * drain rate anymore The following if block is entered only when we
         * have a substantial queue built up (QUEUE_THRESHOLD bytes or more)
         * and we calculate the drain rate for the threshold here.  dq_count is
         * in bytes, time difference in psched_time, hence rate is in
         * bytes/psched_time.
         */
        if (q->vars.dq_count != DQCOUNT_INVALID) {
                q->vars.dq_count += skb->len;

                if (q->vars.dq_count >= QUEUE_THRESHOLD) {
                        psched_time_t now = psched_get_time();
                        u32 dtime = now - q->vars.dq_tstamp;
                        u32 count = q->vars.dq_count << PIE_SCALE;

                        if (dtime == 0)
                                return;

                        count = count / dtime;

                        if (q->vars.avg_dq_rate == 0)
                                q->vars.avg_dq_rate = count;
                        else
                                q->vars.avg_dq_rate =
                                    (q->vars.avg_dq_rate -
                                     (q->vars.avg_dq_rate >> 3)) + (count >> 3);

                        /* If the queue has receded below the threshold, we hold
                         * on to the last drain rate calculated, else we reset
                         * dq_count to 0 to re-enter the if block when the next
                         * packet is dequeued
                         */
                        if (qlen < QUEUE_THRESHOLD)
                                q->vars.dq_count = DQCOUNT_INVALID;
                        else {
                                q->vars.dq_count = 0;
                                q->vars.dq_tstamp = psched_get_time();
                        }

                        if (q->vars.burst_time > 0) {
                                if (q->vars.burst_time > dtime)
                                        q->vars.burst_time -= dtime;
                                else
                                        q->vars.burst_time = 0;
                        }
                }
        }
}

static void calculate_probability(struct Qdisc *sch)
{
        struct pie_sched_data *q = qdisc_priv(sch);
        u32 qlen = sch->qstats.backlog; /* queue size in bytes */
        psched_time_t qdelay = 0;       /* in pschedtime */
        psched_time_t qdelay_old = q->vars.qdelay;      /* in pschedtime */
        s32 delta = 0;          /* determines the change in probability */
        u32 oldprob;
        u32 alpha, beta;
        bool update_prob = true;

        q->vars.qdelay_old = q->vars.qdelay;

        if (q->vars.avg_dq_rate > 0)
                qdelay = (qlen << PIE_SCALE) / q->vars.avg_dq_rate;
        else
                qdelay = 0;

        /* If qdelay is zero and qlen is not, it means qlen is very small, less
         * than dequeue_rate, so we do not update probabilty in this round
         */
        if (qdelay == 0 && qlen != 0)
                update_prob = false;

        /* In the algorithm, alpha and beta are between 0 and 2 with typical
         * value for alpha as 0.125. In this implementation, we use values 0-32
         * passed from user space to represent this. Also, alpha and beta have
         * unit of HZ and need to be scaled before they can used to update
         * probability. alpha/beta are updated locally below by 1) scaling them
         * appropriately 2) scaling down by 16 to come to 0-2 range.
         * Please see paper for details.
         *
         * We scale alpha and beta differently depending on whether we are in
         * light, medium or high dropping mode.
         */
        if (q->vars.prob < MAX_PROB / 100) {
                alpha =
                    (q->params.alpha * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 7;
                beta =
                    (q->params.beta * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 7;
        } else if (q->vars.prob < MAX_PROB / 10) {
                alpha =
                    (q->params.alpha * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 5;
                beta =
                    (q->params.beta * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 5;
        } else {
                alpha =
                    (q->params.alpha * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 4;
                beta =
                    (q->params.beta * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 4;
        }

        /* alpha and beta should be between 0 and 32, in multiples of 1/16 */
        delta += alpha * ((qdelay - q->params.target));
        delta += beta * ((qdelay - qdelay_old));

        oldprob = q->vars.prob;

        /* to ensure we increase probability in steps of no more than 2% */
        if (delta > (s32) (MAX_PROB / (100 / 2)) &&
            q->vars.prob >= MAX_PROB / 10)
                delta = (MAX_PROB / 100) * 2;

        /* Non-linear drop:
         * Tune drop probability to increase quickly for high delays(>= 250ms)
         * 250ms is derived through experiments and provides error protection
         */

        if (qdelay > (PSCHED_NS2TICKS(250 * NSEC_PER_MSEC)))
                delta += MAX_PROB / (100 / 2);

        q->vars.prob += delta;

        if (delta > 0) {
                /* prevent overflow */
                if (q->vars.prob < oldprob) {
                        q->vars.prob = MAX_PROB;
                        /* Prevent normalization error. If probability is at
                         * maximum value already, we normalize it here, and
                         * skip the check to do a non-linear drop in the next
                         * section.
                         */
                        update_prob = false;
                }
        } else {
                /* prevent underflow */
                if (q->vars.prob > oldprob)
                        q->vars.prob = 0;
        }

        /* Non-linear drop in probability: Reduce drop probability quickly if
         * delay is 0 for 2 consecutive Tupdate periods.
         */

        if ((qdelay == 0) && (qdelay_old == 0) && update_prob)
                q->vars.prob = (q->vars.prob * 98) / 100;

        q->vars.qdelay = qdelay;
        q->vars.qlen_old = qlen;

        /* We restart the measurement cycle if the following conditions are met
         * 1. If the delay has been low for 2 consecutive Tupdate periods
         * 2. Calculated drop probability is zero
         * 3. We have atleast one estimate for the avg_dq_rate ie.,
         *    is a non-zero value
         */
        if ((q->vars.qdelay < q->params.target / 2) &&
            (q->vars.qdelay_old < q->params.target / 2) &&
            (q->vars.prob == 0) &&
            (q->vars.avg_dq_rate > 0))
                pie_vars_init(&q->vars);
}

static void pie_timer(struct timer_list *t)
{
        struct pie_sched_data *q = from_timer(q, t, adapt_timer);
        struct Qdisc *sch = q->sch;
        spinlock_t *root_lock = qdisc_lock(qdisc_root_sleeping(sch));

        spin_lock(root_lock);
        calculate_probability(sch);

        /* reset the timer to fire after 'tupdate'. tupdate is in jiffies. */
        if (q->params.tupdate)
                mod_timer(&q->adapt_timer, jiffies + q->params.tupdate);
        spin_unlock(root_lock);

}

static int pie_init(struct Qdisc *sch, struct nlattr *opt,
                    struct netlink_ext_ack *extack)
{
        struct pie_sched_data *q = qdisc_priv(sch);

        pie_params_init(&q->params);
        pie_vars_init(&q->vars);
        sch->limit = q->params.limit;

        q->sch = sch;
        timer_setup(&q->adapt_timer, pie_timer, 0);

        if (opt) {
                int err = pie_change(sch, opt, extack);

                if (err)
                        return err;
        }

        mod_timer(&q->adapt_timer, jiffies + HZ / 2);
        return 0;
}

static int pie_dump(struct Qdisc *sch, struct sk_buff *skb)
{
        struct pie_sched_data *q = qdisc_priv(sch);
        struct nlattr *opts;

        opts = nla_nest_start(skb, TCA_OPTIONS);
        if (opts == NULL)
                goto nla_put_failure;

        /* convert target from pschedtime to us */
        if (nla_put_u32(skb, TCA_PIE_TARGET,
                        ((u32) PSCHED_TICKS2NS(q->params.target)) /
                        NSEC_PER_USEC) ||
            nla_put_u32(skb, TCA_PIE_LIMIT, sch->limit) ||
            nla_put_u32(skb, TCA_PIE_TUPDATE, jiffies_to_usecs(q->params.tupdate)) ||
            nla_put_u32(skb, TCA_PIE_ALPHA, q->params.alpha) ||
            nla_put_u32(skb, TCA_PIE_BETA, q->params.beta) ||
            nla_put_u32(skb, TCA_PIE_ECN, q->params.ecn) ||
            nla_put_u32(skb, TCA_PIE_BYTEMODE, q->params.bytemode))
                goto nla_put_failure;

        return nla_nest_end(skb, opts);

nla_put_failure:
        nla_nest_cancel(skb, opts);
        return -1;

}

static int pie_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
{
        struct pie_sched_data *q = qdisc_priv(sch);
        struct tc_pie_xstats st = {
                .prob           = q->vars.prob,
                .delay          = ((u32) PSCHED_TICKS2NS(q->vars.qdelay)) /
                                   NSEC_PER_USEC,
                /* unscale and return dq_rate in bytes per sec */
                .avg_dq_rate    = q->vars.avg_dq_rate *
                                  (PSCHED_TICKS_PER_SEC) >> PIE_SCALE,
                .packets_in     = q->stats.packets_in,
                .overlimit      = q->stats.overlimit,
                .maxq           = q->stats.maxq,
                .dropped        = q->stats.dropped,
                .ecn_mark       = q->stats.ecn_mark,
        };

        return gnet_stats_copy_app(d, &st, sizeof(st));
}

static struct sk_buff *pie_qdisc_dequeue(struct Qdisc *sch)
{
        struct sk_buff *skb;
        skb = qdisc_dequeue_head(sch);

        if (!skb)
                return NULL;

        pie_process_dequeue(sch, skb);
        return skb;
}

static void pie_reset(struct Qdisc *sch)
{
        struct pie_sched_data *q = qdisc_priv(sch);
        qdisc_reset_queue(sch);
        pie_vars_init(&q->vars);
}

static void pie_destroy(struct Qdisc *sch)
{
        struct pie_sched_data *q = qdisc_priv(sch);
        q->params.tupdate = 0;
        del_timer_sync(&q->adapt_timer);
}

static struct Qdisc_ops pie_qdisc_ops __read_mostly = {
        .id = "pie",
        .priv_size      = sizeof(struct pie_sched_data),
        .enqueue        = pie_qdisc_enqueue,
        .dequeue        = pie_qdisc_dequeue,
        .peek           = qdisc_peek_dequeued,
        .init           = pie_init,
        .destroy        = pie_destroy,
        .reset          = pie_reset,
        .change         = pie_change,
        .dump           = pie_dump,
        .dump_stats     = pie_dump_stats,
        .owner          = THIS_MODULE,
};

static int __init pie_module_init(void)
{
        return register_qdisc(&pie_qdisc_ops);
}

static void __exit pie_module_exit(void)
{
        unregister_qdisc(&pie_qdisc_ops);
}

module_init(pie_module_init);
module_exit(pie_module_exit);

MODULE_DESCRIPTION("Proportional Integral controller Enhanced (PIE) scheduler");
MODULE_AUTHOR("Vijay Subramanian");
MODULE_AUTHOR("Mythili Prabhu");
MODULE_LICENSE("GPL");