Line data    Source code

       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           0 : void dql_completed(struct dql *dql, unsigned int count)
      19             : {
      20           0 :         unsigned int inprogress, prev_inprogress, limit;
      21           0 :         unsigned int ovlimit, completed, num_queued;
      22           0 :         bool all_prev_completed;
      23             : 
      24           0 :         num_queued = READ_ONCE(dql->num_queued);
      25             : 
      26             :         /* Can't complete more than what's in queue */
      27           0 :         BUG_ON(count > num_queued - dql->num_completed);
      28             : 
      29           0 :         completed = dql->num_completed + count;
      30           0 :         limit = dql->limit;
      31           0 :         ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
      32           0 :         inprogress = num_queued - completed;
      33           0 :         prev_inprogress = dql->prev_num_queued - dql->num_completed;
      34           0 :         all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
      35             : 
      36           0 :         if ((ovlimit && !inprogress) ||
      37           0 :             (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           0 :                 limit += POSDIFF(completed, dql->prev_num_queued) +
      54           0 :                      dql->prev_ovlimit;
      55           0 :                 dql->slack_start_time = jiffies;
      56           0 :                 dql->lowest_slack = UINT_MAX;
      57           0 :         } 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           0 :                 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           0 :                 slack = POSDIFF(limit + dql->prev_ovlimit,
      83             :                     2 * (completed - dql->num_completed));
      84           0 :                 slack_last_objs = dql->prev_ovlimit ?
      85           0 :                     POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
      86             : 
      87           0 :                 slack = max(slack, slack_last_objs);
      88             : 
      89           0 :                 if (slack < dql->lowest_slack)
      90           0 :                         dql->lowest_slack = slack;
      91             : 
      92           0 :                 if (time_after(jiffies,
      93             :                                dql->slack_start_time + dql->slack_hold_time)) {
      94           0 :                         limit = POSDIFF(limit, dql->lowest_slack);
      95           0 :                         dql->slack_start_time = jiffies;
      96           0 :                         dql->lowest_slack = UINT_MAX;
      97             :                 }
      98             :         }
      99             : 
     100             :         /* Enforce bounds on limit */
     101           0 :         limit = clamp(limit, dql->min_limit, dql->max_limit);
     102             : 
     103           0 :         if (limit != dql->limit) {
     104           0 :                 dql->limit = limit;
     105           0 :                 ovlimit = 0;
     106             :         }
     107             : 
     108           0 :         dql->adj_limit = limit + completed;
     109           0 :         dql->prev_ovlimit = ovlimit;
     110           0 :         dql->prev_last_obj_cnt = dql->last_obj_cnt;
     111           0 :         dql->num_completed = completed;
     112           0 :         dql->prev_num_queued = num_queued;
     113           0 : }
     114             : EXPORT_SYMBOL(dql_completed);
     115             : 
     116           3 : void dql_reset(struct dql *dql)
     117             : {
     118             :         /* Reset all dynamic values */
     119           3 :         dql->limit = 0;
     120           3 :         dql->num_queued = 0;
     121           3 :         dql->num_completed = 0;
     122           3 :         dql->last_obj_cnt = 0;
     123           3 :         dql->prev_num_queued = 0;
     124           3 :         dql->prev_last_obj_cnt = 0;
     125           3 :         dql->prev_ovlimit = 0;
     126           3 :         dql->lowest_slack = UINT_MAX;
     127           3 :         dql->slack_start_time = jiffies;
     128           0 : }
     129             : EXPORT_SYMBOL(dql_reset);
     130             : 
     131           3 : void dql_init(struct dql *dql, unsigned int hold_time)
     132             : {
     133           3 :         dql->max_limit = DQL_MAX_LIMIT;
     134           3 :         dql->min_limit = 0;
     135           3 :         dql->slack_hold_time = hold_time;
     136           3 :         dql_reset(dql);
     137           3 : }
     138             : EXPORT_SYMBOL(dql_init);