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products/sources/formale Sprachen/C/Linux/lib/ (Open Source Betriebssystem Version 6.17.9^©) Datei vom 24.10.2025 mit Größe 6 kB

Quelle dynamic_queue_limits.c Sprache: C

// SPDX-License-Identifier: GPL-2.0
/*
* Dynamic byte queue limits.  See include/linux/dynamic_queue_limits.h
*
* Copyright (c) 2011, Tom Herbert <therbert@google.com>
*/
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/jiffies.h>
#include <linux/dynamic_queue_limits.h>
#include <linux/compiler.h>
#include <linux/export.h>
#include <trace/events/napi.h>

#define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
#define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)

static void dql_check_stall(struct dql *dql, unsigned short stall_thrs)
{
unsigned long now;

if (!stall_thrs)
  return;

now = jiffies;
/* Check for a potential stall */
if (time_after_eq(now, dql->last_reap + stall_thrs)) {
  unsigned long hist_head, t, start, end;

  /* We are trying to detect a period of at least @stall_thrs
* jiffies without any Tx completions, but during first half
* of which some Tx was posted.
*/
dqs_again:
  hist_head = READ_ONCE(dql->history_head);
  /* pairs with smp_wmb() in dql_queued() */
  smp_rmb();

  /* Get the previous entry in the ring buffer, which is the
* oldest sample.
*/
  start = (hist_head - DQL_HIST_LEN + 1) * BITS_PER_LONG;

  /* Advance start to continue from the last reap time */
  if (time_before(start, dql->last_reap + 1))
   start = dql->last_reap + 1;

  /* Newest sample we should have already seen a completion for */
  end = hist_head * BITS_PER_LONG + (BITS_PER_LONG - 1);

  /* Shrink the search space to [start, (now - start_thrs/2)] if
* `end` is beyond the stall zone
*/
  if (time_before(now, end + stall_thrs / 2))
   end = now - stall_thrs / 2;

  /* Search for the queued time in [t, end] */
  for (t = start; time_before_eq(t, end); t++)
   if (test_bit(t % (DQL_HIST_LEN * BITS_PER_LONG),
         dql->history))
    break;

  /* Variable t contains the time of the queue */
  if (!time_before_eq(t, end))
   goto no_stall;

  /* The ring buffer was modified in the meantime, retry */
  if (hist_head != READ_ONCE(dql->history_head))
   goto dqs_again;

  dql->stall_cnt++;
  dql->stall_max = max_t(unsigned short, dql->stall_max, now - t);

  trace_dql_stall_detected(dql->stall_thrs, now - t,
      dql->last_reap, dql->history_head,
      now, dql->history);
}
no_stall:
dql->last_reap = now;
}

/* Records completed count and recalculates the queue limit */
void dql_completed(struct dql *dql, unsigned int count)
{
unsigned int inprogress, prev_inprogress, limit;
unsigned int ovlimit, completed, num_queued;
unsigned short stall_thrs;
bool all_prev_completed;

num_queued = READ_ONCE(dql->num_queued);
/* Read stall_thrs in advance since it belongs to the same (first)
* cache line as ->num_queued. This way, dql_check_stall() does not
* need to touch the first cache line again later, reducing the window
* of possible false sharing.
*/
stall_thrs = READ_ONCE(dql->stall_thrs);

/* Can't complete more than what's in queue */
BUG_ON(count > num_queued - dql->num_completed);

completed = dql->num_completed + count;
limit = dql->limit;
ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
inprogress = num_queued - completed;
prev_inprogress = dql->prev_num_queued - dql->num_completed;
all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);

if ((ovlimit && !inprogress) ||
     (dql->prev_ovlimit && all_prev_completed)) {
  /*
* Queue considered starved if:
*   - The queue was over-limit in the last interval,
*     and there is no more data in the queue.
*  OR
*   - The queue was over-limit in the previous interval and
*     when enqueuing it was possible that all queued data
*     had been consumed.  This covers the case when queue
*     may have becomes starved between completion processing
*     running and next time enqueue was scheduled.
*
*     When queue is starved increase the limit by the amount
*     of bytes both sent and completed in the last interval,
*     plus any previous over-limit.
*/
  limit += POSDIFF(completed, dql->prev_num_queued) +
       dql->prev_ovlimit;
  dql->slack_start_time = jiffies;
  dql->lowest_slack = UINT_MAX;
} else if (inprogress && prev_inprogress && !all_prev_completed) {
  /*
* Queue was not starved, check if the limit can be decreased.
* A decrease is only considered if the queue has been busy in
* the whole interval (the check above).
*
* If there is slack, the amount of excess data queued above
* the amount needed to prevent starvation, the queue limit
* can be decreased.  To avoid hysteresis we consider the
* minimum amount of slack found over several iterations of the
* completion routine.
*/
  unsigned int slack, slack_last_objs;

  /*
* Slack is the maximum of
*   - The queue limit plus previous over-limit minus twice
*     the number of objects completed.  Note that two times
*     number of completed bytes is a basis for an upper bound
*     of the limit.
*   - Portion of objects in the last queuing operation that
*     was not part of non-zero previous over-limit.  That is
*     "round down" by non-overlimit portion of the last
*     queueing operation.
*/
  slack = POSDIFF(limit + dql->prev_ovlimit,
      2 * (completed - dql->num_completed));
  slack_last_objs = dql->prev_ovlimit ?
      POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;

  slack = max(slack, slack_last_objs);

  if (slack < dql->lowest_slack)
   dql->lowest_slack = slack;

  if (time_after(jiffies,
          dql->slack_start_time + dql->slack_hold_time)) {
   limit = POSDIFF(limit, dql->lowest_slack);
   dql->slack_start_time = jiffies;
   dql->lowest_slack = UINT_MAX;
  }
}

/* Enforce bounds on limit */
limit = clamp(limit, dql->min_limit, dql->max_limit);

if (limit != dql->limit) {
  dql->limit = limit;
  ovlimit = 0;
}

dql->adj_limit = limit + completed;
dql->prev_ovlimit = ovlimit;
dql->prev_last_obj_cnt = READ_ONCE(dql->last_obj_cnt);
dql->num_completed = completed;
dql->prev_num_queued = num_queued;

dql_check_stall(dql, stall_thrs);
}
EXPORT_SYMBOL(dql_completed);

void dql_reset(struct dql *dql)
{
/* Reset all dynamic values */
dql->limit = dql->min_limit;
dql->num_queued = 0;
dql->num_completed = 0;
dql->last_obj_cnt = 0;
dql->prev_num_queued = 0;
dql->prev_last_obj_cnt = 0;
dql->prev_ovlimit = 0;
dql->lowest_slack = UINT_MAX;
dql->slack_start_time = jiffies;

dql->last_reap = jiffies;
dql->history_head = jiffies / BITS_PER_LONG;
memset(dql->history, 0, sizeof(dql->history));
}
EXPORT_SYMBOL(dql_reset);

void dql_init(struct dql *dql, unsigned int hold_time)
{
dql->max_limit = DQL_MAX_LIMIT;
dql->min_limit = 0;
dql->slack_hold_time = hold_time;
dql->stall_thrs = 0;
dql_reset(dql);
}
EXPORT_SYMBOL(dql_init);

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