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