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       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                 :            :         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         [ #  # ]:          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 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                 :          0 :                 slack = POSDIFF(limit + dql->prev_ovlimit,
      83                 :            :                     2 * (completed - dql->num_completed));
      84                 :            :                 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                 :            :                 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                 :          0 : void dql_reset(struct dql *dql)
     117                 :            : {
     118                 :            :         /* Reset all dynamic values */
     119                 :        621 :         dql->limit = 0;
     120                 :        621 :         dql->num_queued = 0;
     121                 :        621 :         dql->num_completed = 0;
     122                 :        621 :         dql->last_obj_cnt = 0;
     123                 :        621 :         dql->prev_num_queued = 0;
     124                 :        621 :         dql->prev_last_obj_cnt = 0;
     125                 :        621 :         dql->prev_ovlimit = 0;
     126                 :        621 :         dql->lowest_slack = UINT_MAX;
     127                 :        621 :         dql->slack_start_time = jiffies;
     128                 :          0 : }
     129                 :            : EXPORT_SYMBOL(dql_reset);
     130                 :            : 
     131                 :        621 : void dql_init(struct dql *dql, unsigned int hold_time)
     132                 :            : {
     133                 :        621 :         dql->max_limit = DQL_MAX_LIMIT;
     134                 :        621 :         dql->min_limit = 0;
     135                 :        621 :         dql->slack_hold_time = hold_time;
     136                 :            :         dql_reset(dql);
     137                 :        621 : }
     138                 :            : EXPORT_SYMBOL(dql_init);