Quelle dynamic_queue_limits.h Sprache: C

/* SPDX-License-Identifier: GPL-2.0 */
/*
* Dynamic queue limits (dql) - Definitions
*
* Copyright (c) 2011, Tom Herbert <therbert@google.com>
*
* This header file contains the definitions for dynamic queue limits (dql).
* dql would be used in conjunction with a producer/consumer type queue
* (possibly a HW queue).  Such a queue would have these general properties:
*
*   1) Objects are queued up to some limit specified as number of objects.
*   2) Periodically a completion process executes which retires consumed
*      objects.
*   3) Starvation occurs when limit has been reached, all queued data has
*      actually been consumed, but completion processing has not yet run
*      so queuing new data is blocked.
*   4) Minimizing the amount of queued data is desirable.
*
* The goal of dql is to calculate the limit as the minimum number of objects
* needed to prevent starvation.
*
* The primary functions of dql are:
*    dql_queued - called when objects are enqueued to record number of objects
*    dql_avail - returns how many objects are available to be queued based
*      on the object limit and how many objects are already enqueued
*    dql_completed - called at completion time to indicate how many objects
*      were retired from the queue
*
* The dql implementation does not implement any locking for the dql data
* structures, the higher layer should provide this.  dql_queued should
* be serialized to prevent concurrent execution of the function; this
* is also true for  dql_completed.  However, dql_queued and dlq_completed  can
* be executed concurrently (i.e. they can be protected by different locks).
*/

#ifndef _LINUX_DQL_H
#define _LINUX_DQL_H

#ifdef __KERNEL__

#include <linux/bitops.h>
#include <asm/bug.h>

#define DQL_HIST_LEN  4
#define DQL_HIST_ENT(dql, idx) ((dql)->history[(idx) % DQL_HIST_LEN])

struct dql {
/* Fields accessed in enqueue path (dql_queued) */
unsigned int num_queued;  /* Total ever queued */
unsigned int adj_limit;  /* limit + num_completed */
unsigned int last_obj_cnt;  /* Count at last queuing */

/* Stall threshold (in jiffies), defined by user */
unsigned short stall_thrs;

unsigned long history_head;  /* top 58 bits of jiffies */
/* stall entries, a bit per entry */
unsigned long history[DQL_HIST_LEN];

/* Fields accessed only by completion path (dql_completed) */

unsigned int limit ____cacheline_aligned_in_smp; /* Current limit */
unsigned int num_completed;  /* Total ever completed */

unsigned int prev_ovlimit;  /* Previous over limit */
unsigned int prev_num_queued; /* Previous queue total */
unsigned int prev_last_obj_cnt; /* Previous queuing cnt */

unsigned int lowest_slack;  /* Lowest slack found */
unsigned long slack_start_time; /* Time slacks seen */

/* Configuration */
unsigned int max_limit;  /* Max limit */
unsigned int min_limit;  /* Minimum limit */
unsigned int slack_hold_time; /* Time to measure slack */

/* Longest stall detected, reported to user */
unsigned short stall_max;
unsigned long last_reap;  /* Last reap (in jiffies) */
unsigned long stall_cnt;  /* Number of stalls */
};

/* Set some static maximums */
#define DQL_MAX_OBJECT (UINT_MAX / 16)
#define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT)

/* Populate the bitmap to be processed later in dql_check_stall() */
static inline void dql_queue_stall(struct dql *dql)
{
unsigned long map, now, now_hi, i;

now = jiffies;
now_hi = now / BITS_PER_LONG;

/* The following code set a bit in the ring buffer, where each
* bit trackes time the packet was queued. The dql->history buffer
* tracks DQL_HIST_LEN * BITS_PER_LONG time (jiffies) slot
*/
if (unlikely(now_hi != dql->history_head)) {
  /* About to reuse slots, clear them */
  for (i = 0; i < DQL_HIST_LEN; i++) {
   /* Multiplication masks high bits */
   if (now_hi * BITS_PER_LONG ==
       (dql->history_head + i) * BITS_PER_LONG)
    break;
   DQL_HIST_ENT(dql, dql->history_head + i + 1) = 0;
  }
  /* pairs with smp_rmb() in dql_check_stall() */
  smp_wmb();
  WRITE_ONCE(dql->history_head, now_hi);
}

/* __set_bit() does not guarantee WRITE_ONCE() semantics */
map = DQL_HIST_ENT(dql, now_hi);

/* Populate the history with an entry (bit) per queued */
if (!(map & BIT_MASK(now)))
  WRITE_ONCE(DQL_HIST_ENT(dql, now_hi), map | BIT_MASK(now));
}

/*
* Record number of objects queued. Assumes that caller has already checked
* availability in the queue with dql_avail.
*/
static inline void dql_queued(struct dql *dql, unsigned int count)
{
if (WARN_ON_ONCE(count > DQL_MAX_OBJECT))
  return;

WRITE_ONCE(dql->last_obj_cnt, count);

/* We want to force a write first, so that cpu do not attempt
* to get cache line containing last_obj_cnt, num_queued, adj_limit
* in Shared state, but directly does a Request For Ownership
* It is only a hint, we use barrier() only.
*/
barrier();

dql->num_queued += count;

/* Only populate stall information if the threshold is set */
if (READ_ONCE(dql->stall_thrs))
  dql_queue_stall(dql);
}

/* Returns how many objects can be queued, < 0 indicates over limit. */
static inline int dql_avail(const struct dql *dql)
{
return READ_ONCE(dql->adj_limit) - READ_ONCE(dql->num_queued);
}

/* Record number of completed objects and recalculate the limit. */
void dql_completed(struct dql *dql, unsigned int count);

/* Reset dql state */
void dql_reset(struct dql *dql);

/* Initialize dql state */
void dql_init(struct dql *dql, unsigned int hold_time);

#endif /* _KERNEL_ */

#endif /* _LINUX_DQL_H */

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