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![Breno Leitao](/assets/img/avatar_default.png)
With the previous change, struct dqs->stall_thrs will be in the hot path (at queue side), even if DQS is disabled. The other fields accessed in this function (last_obj_cnt and num_queued) are in the first cache line, let's move this field (stall_thrs) to the very first cache line, since there is a hole there. This does not change the structure size, since it moves an short (2 bytes) to 4-bytes whole in the first cache line. This is the new structure format now: struct dql { unsigned int num_queued; unsigned int last_obj_cnt; ... short unsigned int stall_thrs; /* XXX 2 bytes hole, try to pack */ ... /* --- cacheline 1 boundary (64 bytes) --- */ ... /* Longest stall detected, reported to user */ short unsigned int stall_max; /* XXX 2 bytes hole, try to pack */ }; Also, read the stall_thrs (now in the very first cache line) earlier, together with dql->num_queued (also in the first cache line). Suggested-by: Jakub Kicinski <kuba@kernel.org> Suggested-by: Eric Dumazet <edumazet@google.com> Signed-off-by: Breno Leitao <leitao@debian.org> Link: https://lore.kernel.org/r/20240411192241.2498631-5-leitao@debian.org Signed-off-by: Jakub Kicinski <kuba@kernel.org>
218 lines
6.6 KiB
C
218 lines
6.6 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* Dynamic byte queue limits. See include/linux/dynamic_queue_limits.h
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*
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* Copyright (c) 2011, Tom Herbert <therbert@google.com>
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*/
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#include <linux/types.h>
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#include <linux/kernel.h>
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#include <linux/jiffies.h>
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#include <linux/dynamic_queue_limits.h>
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#include <linux/compiler.h>
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#include <linux/export.h>
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#include <trace/events/napi.h>
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#define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
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#define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)
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static void dql_check_stall(struct dql *dql, unsigned short stall_thrs)
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{
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unsigned long now;
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if (!stall_thrs)
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return;
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now = jiffies;
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/* Check for a potential stall */
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if (time_after_eq(now, dql->last_reap + stall_thrs)) {
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unsigned long hist_head, t, start, end;
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/* We are trying to detect a period of at least @stall_thrs
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* jiffies without any Tx completions, but during first half
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* of which some Tx was posted.
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*/
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dqs_again:
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hist_head = READ_ONCE(dql->history_head);
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/* pairs with smp_wmb() in dql_queued() */
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smp_rmb();
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/* Get the previous entry in the ring buffer, which is the
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* oldest sample.
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*/
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start = (hist_head - DQL_HIST_LEN + 1) * BITS_PER_LONG;
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/* Advance start to continue from the last reap time */
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if (time_before(start, dql->last_reap + 1))
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start = dql->last_reap + 1;
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/* Newest sample we should have already seen a completion for */
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end = hist_head * BITS_PER_LONG + (BITS_PER_LONG - 1);
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/* Shrink the search space to [start, (now - start_thrs/2)] if
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* `end` is beyond the stall zone
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*/
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if (time_before(now, end + stall_thrs / 2))
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end = now - stall_thrs / 2;
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/* Search for the queued time in [t, end] */
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for (t = start; time_before_eq(t, end); t++)
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if (test_bit(t % (DQL_HIST_LEN * BITS_PER_LONG),
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dql->history))
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break;
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/* Variable t contains the time of the queue */
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if (!time_before_eq(t, end))
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goto no_stall;
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/* The ring buffer was modified in the meantime, retry */
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if (hist_head != READ_ONCE(dql->history_head))
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goto dqs_again;
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dql->stall_cnt++;
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dql->stall_max = max_t(unsigned short, dql->stall_max, now - t);
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trace_dql_stall_detected(dql->stall_thrs, now - t,
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dql->last_reap, dql->history_head,
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now, dql->history);
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}
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no_stall:
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dql->last_reap = now;
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}
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/* Records completed count and recalculates the queue limit */
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void dql_completed(struct dql *dql, unsigned int count)
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{
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unsigned int inprogress, prev_inprogress, limit;
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unsigned int ovlimit, completed, num_queued;
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unsigned short stall_thrs;
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bool all_prev_completed;
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num_queued = READ_ONCE(dql->num_queued);
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/* Read stall_thrs in advance since it belongs to the same (first)
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* cache line as ->num_queued. This way, dql_check_stall() does not
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* need to touch the first cache line again later, reducing the window
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* of possible false sharing.
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*/
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stall_thrs = READ_ONCE(dql->stall_thrs);
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/* Can't complete more than what's in queue */
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BUG_ON(count > num_queued - dql->num_completed);
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completed = dql->num_completed + count;
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limit = dql->limit;
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ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
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inprogress = num_queued - completed;
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prev_inprogress = dql->prev_num_queued - dql->num_completed;
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all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
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if ((ovlimit && !inprogress) ||
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(dql->prev_ovlimit && all_prev_completed)) {
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/*
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* Queue considered starved if:
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* - The queue was over-limit in the last interval,
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* and there is no more data in the queue.
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* OR
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* - The queue was over-limit in the previous interval and
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* when enqueuing it was possible that all queued data
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* had been consumed. This covers the case when queue
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* may have becomes starved between completion processing
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* running and next time enqueue was scheduled.
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*
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* When queue is starved increase the limit by the amount
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* of bytes both sent and completed in the last interval,
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* plus any previous over-limit.
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*/
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limit += POSDIFF(completed, dql->prev_num_queued) +
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dql->prev_ovlimit;
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dql->slack_start_time = jiffies;
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dql->lowest_slack = UINT_MAX;
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} else if (inprogress && prev_inprogress && !all_prev_completed) {
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/*
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* Queue was not starved, check if the limit can be decreased.
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* A decrease is only considered if the queue has been busy in
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* the whole interval (the check above).
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*
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* If there is slack, the amount of excess data queued above
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* the amount needed to prevent starvation, the queue limit
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* can be decreased. To avoid hysteresis we consider the
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* minimum amount of slack found over several iterations of the
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* completion routine.
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*/
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unsigned int slack, slack_last_objs;
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/*
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* Slack is the maximum of
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* - The queue limit plus previous over-limit minus twice
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* the number of objects completed. Note that two times
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* number of completed bytes is a basis for an upper bound
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* of the limit.
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* - Portion of objects in the last queuing operation that
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* was not part of non-zero previous over-limit. That is
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* "round down" by non-overlimit portion of the last
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* queueing operation.
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*/
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slack = POSDIFF(limit + dql->prev_ovlimit,
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2 * (completed - dql->num_completed));
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slack_last_objs = dql->prev_ovlimit ?
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POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
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slack = max(slack, slack_last_objs);
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if (slack < dql->lowest_slack)
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dql->lowest_slack = slack;
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if (time_after(jiffies,
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dql->slack_start_time + dql->slack_hold_time)) {
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limit = POSDIFF(limit, dql->lowest_slack);
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dql->slack_start_time = jiffies;
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dql->lowest_slack = UINT_MAX;
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}
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}
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/* Enforce bounds on limit */
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limit = clamp(limit, dql->min_limit, dql->max_limit);
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if (limit != dql->limit) {
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dql->limit = limit;
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ovlimit = 0;
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}
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dql->adj_limit = limit + completed;
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dql->prev_ovlimit = ovlimit;
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dql->prev_last_obj_cnt = dql->last_obj_cnt;
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dql->num_completed = completed;
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dql->prev_num_queued = num_queued;
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dql_check_stall(dql, stall_thrs);
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}
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EXPORT_SYMBOL(dql_completed);
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void dql_reset(struct dql *dql)
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{
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/* Reset all dynamic values */
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dql->limit = 0;
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dql->num_queued = 0;
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dql->num_completed = 0;
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dql->last_obj_cnt = 0;
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dql->prev_num_queued = 0;
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dql->prev_last_obj_cnt = 0;
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dql->prev_ovlimit = 0;
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dql->lowest_slack = UINT_MAX;
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dql->slack_start_time = jiffies;
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dql->last_reap = jiffies;
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dql->history_head = jiffies / BITS_PER_LONG;
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memset(dql->history, 0, sizeof(dql->history));
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}
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EXPORT_SYMBOL(dql_reset);
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void dql_init(struct dql *dql, unsigned int hold_time)
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{
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dql->max_limit = DQL_MAX_LIMIT;
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dql->min_limit = 0;
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dql->slack_hold_time = hold_time;
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dql->stall_thrs = 0;
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dql_reset(dql);
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}
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EXPORT_SYMBOL(dql_init);
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