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