lib/dynamic_queue_limits.c

   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 */
  18 void 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 execess data queued above
  64                  * the 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 }
 114 EXPORT_SYMBOL(dql_completed);
 115
 116 void 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 }
 129 EXPORT_SYMBOL(dql_reset);
 130
 131 int dql_init(struct dql *dql, unsigned 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         return 0;
 138 }
 139 EXPORT_SYMBOL(dql_init);