lib/dynamic_queue_limits.c

   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 #include <trace/events/napi.h>
  14
  15 #define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
  16 #define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)
  17
  18 static void dql_check_stall(struct dql *dql, unsigned short stall_thrs)
  19 {
  20         unsigned long now;
  21
  22         if (!stall_thrs)
  23                 return;
  24
  25         now = jiffies;
  26         /* Check for a potential stall */
  27         if (time_after_eq(now, dql->last_reap + stall_thrs)) {
  28                 unsigned long hist_head, t, start, end;
  29
  30                 /* We are trying to detect a period of at least @stall_thrs
  31                  * jiffies without any Tx completions, but during first half
  32                  * of which some Tx was posted.
  33                  */
  34 dqs_again:
  35                 hist_head = READ_ONCE(dql->history_head);
  36                 /* pairs with smp_wmb() in dql_queued() */
  37                 smp_rmb();
  38
  39                 /* Get the previous entry in the ring buffer, which is the
  40                  * oldest sample.
  41                  */
  42                 start = (hist_head - DQL_HIST_LEN + 1) * BITS_PER_LONG;
  43
  44                 /* Advance start to continue from the last reap time */
  45                 if (time_before(start, dql->last_reap + 1))
  46                         start = dql->last_reap + 1;
  47
  48                 /* Newest sample we should have already seen a completion for */
  49                 end = hist_head * BITS_PER_LONG + (BITS_PER_LONG - 1);
  50
  51                 /* Shrink the search space to [start, (now - start_thrs/2)] if
  52                  * `end` is beyond the stall zone
  53                  */
  54                 if (time_before(now, end + stall_thrs / 2))
  55                         end = now - stall_thrs / 2;
  56
  57                 /* Search for the queued time in [t, end] */
  58                 for (t = start; time_before_eq(t, end); t++)
  59                         if (test_bit(t % (DQL_HIST_LEN * BITS_PER_LONG),
  60                                      dql->history))
  61                                 break;
  62
  63                 /* Variable t contains the time of the queue */
  64                 if (!time_before_eq(t, end))
  65                         goto no_stall;
  66
  67                 /* The ring buffer was modified in the meantime, retry */
  68                 if (hist_head != READ_ONCE(dql->history_head))
  69                         goto dqs_again;
  70
  71                 dql->stall_cnt++;
  72                 dql->stall_max = max_t(unsigned short, dql->stall_max, now - t);
  73
  74                 trace_dql_stall_detected(dql->stall_thrs, now - t,
  75                                          dql->last_reap, dql->history_head,
  76                                          now, dql->history);
  77         }
  78 no_stall:
  79         dql->last_reap = now;
  80 }
  81
  82 /* Records completed count and recalculates the queue limit */
  83 void dql_completed(struct dql *dql, unsigned int count)
  84 {
  85         unsigned int inprogress, prev_inprogress, limit;
  86         unsigned int ovlimit, completed, num_queued;
  87         unsigned short stall_thrs;
  88         bool all_prev_completed;
  89
  90         num_queued = READ_ONCE(dql->num_queued);
  91         /* Read stall_thrs in advance since it belongs to the same (first)
  92          * cache line as ->num_queued. This way, dql_check_stall() does not
  93          * need to touch the first cache line again later, reducing the window
  94          * of possible false sharing.
  95          */
  96         stall_thrs = READ_ONCE(dql->stall_thrs);
  97
  98         /* Can't complete more than what's in queue */
  99         BUG_ON(count > num_queued - dql->num_completed);
 100
 101         completed = dql->num_completed + count;
 102         limit = dql->limit;
 103         ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
 104         inprogress = num_queued - completed;
 105         prev_inprogress = dql->prev_num_queued - dql->num_completed;
 106         all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
 107
 108         if ((ovlimit && !inprogress) ||
 109             (dql->prev_ovlimit && all_prev_completed)) {
 110                 /*
 111                  * Queue considered starved if:
 112                  *   - The queue was over-limit in the last interval,
 113                  *     and there is no more data in the queue.
 114                  *  OR
 115                  *   - The queue was over-limit in the previous interval and
 116                  *     when enqueuing it was possible that all queued data
 117                  *     had been consumed.  This covers the case when queue
 118                  *     may have becomes starved between completion processing
 119                  *     running and next time enqueue was scheduled.
 120                  *
 121                  *     When queue is starved increase the limit by the amount
 122                  *     of bytes both sent and completed in the last interval,
 123                  *     plus any previous over-limit.
 124                  */
 125                 limit += POSDIFF(completed, dql->prev_num_queued) +
 126                      dql->prev_ovlimit;
 127                 dql->slack_start_time = jiffies;
 128                 dql->lowest_slack = UINT_MAX;
 129         } else if (inprogress && prev_inprogress && !all_prev_completed) {
 130                 /*
 131                  * Queue was not starved, check if the limit can be decreased.
 132                  * A decrease is only considered if the queue has been busy in
 133                  * the whole interval (the check above).
 134                  *
 135                  * If there is slack, the amount of excess data queued above
 136                  * the amount needed to prevent starvation, the queue limit
 137                  * can be decreased.  To avoid hysteresis we consider the
 138                  * minimum amount of slack found over several iterations of the
 139                  * completion routine.
 140                  */
 141                 unsigned int slack, slack_last_objs;
 142
 143                 /*
 144                  * Slack is the maximum of
 145                  *   - The queue limit plus previous over-limit minus twice
 146                  *     the number of objects completed.  Note that two times
 147                  *     number of completed bytes is a basis for an upper bound
 148                  *     of the limit.
 149                  *   - Portion of objects in the last queuing operation that
 150                  *     was not part of non-zero previous over-limit.  That is
 151                  *     "round down" by non-overlimit portion of the last
 152                  *     queueing operation.
 153                  */
 154                 slack = POSDIFF(limit + dql->prev_ovlimit,
 155                     2 * (completed - dql->num_completed));
 156                 slack_last_objs = dql->prev_ovlimit ?
 157                     POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
 158
 159                 slack = max(slack, slack_last_objs);
 160
 161                 if (slack < dql->lowest_slack)
 162                         dql->lowest_slack = slack;
 163
 164                 if (time_after(jiffies,
 165                                dql->slack_start_time + dql->slack_hold_time)) {
 166                         limit = POSDIFF(limit, dql->lowest_slack);
 167                         dql->slack_start_time = jiffies;
 168                         dql->lowest_slack = UINT_MAX;
 169                 }
 170         }
 171
 172         /* Enforce bounds on limit */
 173         limit = clamp(limit, dql->min_limit, dql->max_limit);
 174
 175         if (limit != dql->limit) {
 176                 dql->limit = limit;
 177                 ovlimit = 0;
 178         }
 179
 180         dql->adj_limit = limit + completed;
 181         dql->prev_ovlimit = ovlimit;
 182         dql->prev_last_obj_cnt = READ_ONCE(dql->last_obj_cnt);
 183         dql->num_completed = completed;
 184         dql->prev_num_queued = num_queued;
 185
 186         dql_check_stall(dql, stall_thrs);
 187 }
 188 EXPORT_SYMBOL(dql_completed);
 189
 190 void dql_reset(struct dql *dql)
 191 {
 192         /* Reset all dynamic values */
 193         dql->limit = 0;
 194         dql->num_queued = 0;
 195         dql->num_completed = 0;
 196         dql->last_obj_cnt = 0;
 197         dql->prev_num_queued = 0;
 198         dql->prev_last_obj_cnt = 0;
 199         dql->prev_ovlimit = 0;
 200         dql->lowest_slack = UINT_MAX;
 201         dql->slack_start_time = jiffies;
 202
 203         dql->last_reap = jiffies;
 204         dql->history_head = jiffies / BITS_PER_LONG;
 205         memset(dql->history, 0, sizeof(dql->history));
 206 }
 207 EXPORT_SYMBOL(dql_reset);
 208
 209 void dql_init(struct dql *dql, unsigned int hold_time)
 210 {
 211         dql->max_limit = DQL_MAX_LIMIT;
 212         dql->min_limit = 0;
 213         dql->slack_hold_time = hold_time;
 214         dql->stall_thrs = 0;
 215         dql_reset(dql);
 216 }
 217 EXPORT_SYMBOL(dql_init);