LXR linux/lib/dynamic_queue

   1// SPDX-License-Identifier: GPL-2.0
   2/*
   3 * Dynamic byte queue limits.  See include/linux/dynamic_queue_limits.h
   4 *
   5 * Copyright (c) 2011, Tom Herbert <therbert@google.com>
   6 */
   7#include <linux/types.h>
   8#include <linux/kernel.h>
   9#include <linux/jiffies.h>
  10#include <linux/dynamic_queue_limits.h>
  11#include <linux/compiler.h>
  12#include <linux/export.h>
  13
  14#define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
  15#define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)
  16
  17/* Records completed count and recalculates the queue limit */
  18void dql_completed(struct dql *dql, unsigned int count)
  19{
  20        unsigned int inprogress, prev_inprogress, limit;
  21        unsigned int ovlimit, completed, num_queued;
  22        bool all_prev_completed;
  23
  24        num_queued = READ_ONCE(dql->num_queued);
  25
  26        /* Can't complete more than what's in queue */
  27        BUG_ON(count > num_queued - dql->num_completed);
  28
  29        completed = dql->num_completed + count;
  30        limit = dql->limit;
  31        ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
  32        inprogress = num_queued - completed;
  33        prev_inprogress = dql->prev_num_queued - dql->num_completed;
  34        all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
  35
  36        if ((ovlimit && !inprogress) ||
  37            (dql->prev_ovlimit && all_prev_completed)) {
  38                /*
  39                 * Queue considered starved if:
  40                 *   - The queue was over-limit in the last interval,
  41                 *     and there is no more data in the queue.
  42                 *  OR
  43                 *   - The queue was over-limit in the previous interval and
  44                 *     when enqueuing it was possible that all queued data
  45                 *     had been consumed.  This covers the case when queue
  46                 *     may have becomes starved between completion processing
  47                 *     running and next time enqueue was scheduled.
  48                 *
  49                 *     When queue is starved increase the limit by the amount
  50                 *     of bytes both sent and completed in the last interval,
  51                 *     plus any previous over-limit.
  52                 */
  53                limit += POSDIFF(completed, dql->prev_num_queued) +
  54                     dql->prev_ovlimit;
  55                dql->slack_start_time = jiffies;
  56                dql->lowest_slack = UINT_MAX;
  57        } else if (inprogress && prev_inprogress && !all_prev_completed) {
  58                /*
  59                 * Queue was not starved, check if the limit can be decreased.
  60                 * A decrease is only considered if the queue has been busy in
  61                 * the whole interval (the check above).
  62                 *
  63                 * If there is slack, the amount of excess data queued above
  64                 * the amount needed to prevent starvation, the queue limit
  65                 * can be decreased.  To avoid hysteresis we consider the
  66                 * minimum amount of slack found over several iterations of the
  67                 * completion routine.
  68                 */
  69                unsigned int slack, slack_last_objs;
  70
  71                /*
  72                 * Slack is the maximum of
  73                 *   - The queue limit plus previous over-limit minus twice
  74                 *     the number of objects completed.  Note that two times
  75                 *     number of completed bytes is a basis for an upper bound
  76                 *     of the limit.
  77                 *   - Portion of objects in the last queuing operation that
  78                 *     was not part of non-zero previous over-limit.  That is
  79                 *     "round down" by non-overlimit portion of the last
  80                 *     queueing operation.
  81                 */
  82                slack = POSDIFF(limit + dql->prev_ovlimit,
  83                    2 * (completed - dql->num_completed));
  84                slack_last_objs = dql->prev_ovlimit ?
  85                    POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
  86
  87                slack = max(slack, slack_last_objs);
  88
  89                if (slack < dql->lowest_slack)
  90                        dql->lowest_slack = slack;
  91
  92                if (time_after(jiffies,
  93                               dql->slack_start_time + dql->slack_hold_time)) {
  94                        limit = POSDIFF(limit, dql->lowest_slack);
  95                        dql->slack_start_time = jiffies;
  96                        dql->lowest_slack = UINT_MAX;
  97                }
  98        }
  99
 100        /* Enforce bounds on limit */
 101        limit = clamp(limit, dql->min_limit, dql->max_limit);
 102
 103        if (limit != dql->limit) {
 104                dql->limit = limit;
 105                ovlimit = 0;
 106        }
 107
 108        dql->adj_limit = limit + completed;
 109        dql->prev_ovlimit = ovlimit;
 110        dql->prev_last_obj_cnt = dql->last_obj_cnt;
 111        dql->num_completed = completed;
 112        dql->prev_num_queued = num_queued;
 113}
 114EXPORT_SYMBOL(dql_completed);
 115
 116void dql_reset(struct dql *dql)
 117{
 118        /* Reset all dynamic values */
 119        dql->limit = 0;
 120        dql->num_queued = 0;
 121        dql->num_completed = 0;
 122        dql->last_obj_cnt = 0;
 123        dql->prev_num_queued = 0;
 124        dql->prev_last_obj_cnt = 0;
 125        dql->prev_ovlimit = 0;
 126        dql->lowest_slack = UINT_MAX;
 127        dql->slack_start_time = jiffies;
 128}
 129EXPORT_SYMBOL(dql_reset);
 130
 131void dql_init(struct dql *dql, unsigned int hold_time)
 132{
 133        dql->max_limit = DQL_MAX_LIMIT;
 134        dql->min_limit = 0;
 135        dql->slack_hold_time = hold_time;
 136        dql_reset(dql);
 137}
 138EXPORT_SYMBOL(dql_init);
 139