lib/dynamic_queue_limits.c

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