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