LXR linux/include/linux/dynamic_queue

   1/* SPDX-License-Identifier: GPL-2.0 */
   2/*
   3 * Dynamic queue limits (dql) - Definitions
   4 *
   5 * Copyright (c) 2011, Tom Herbert <therbert@google.com>
   6 *
   7 * This header file contains the definitions for dynamic queue limits (dql).
   8 * dql would be used in conjunction with a producer/consumer type queue
   9 * (possibly a HW queue).  Such a queue would have these general properties:
  10 *
  11 *   1) Objects are queued up to some limit specified as number of objects.
  12 *   2) Periodically a completion process executes which retires consumed
  13 *      objects.
  14 *   3) Starvation occurs when limit has been reached, all queued data has
  15 *      actually been consumed, but completion processing has not yet run
  16 *      so queuing new data is blocked.
  17 *   4) Minimizing the amount of queued data is desirable.
  18 *
  19 * The goal of dql is to calculate the limit as the minimum number of objects
  20 * needed to prevent starvation.
  21 *
  22 * The primary functions of dql are:
  23 *    dql_queued - called when objects are enqueued to record number of objects
  24 *    dql_avail - returns how many objects are available to be queued based
  25 *      on the object limit and how many objects are already enqueued
  26 *    dql_completed - called at completion time to indicate how many objects
  27 *      were retired from the queue
  28 *
  29 * The dql implementation does not implement any locking for the dql data
  30 * structures, the higher layer should provide this.  dql_queued should
  31 * be serialized to prevent concurrent execution of the function; this
  32 * is also true for  dql_completed.  However, dql_queued and dlq_completed  can
  33 * be executed concurrently (i.e. they can be protected by different locks).
  34 */
  35
  36#ifndef _LINUX_DQL_H
  37#define _LINUX_DQL_H
  38
  39#ifdef __KERNEL__
  40
  41struct dql {
  42        /* Fields accessed in enqueue path (dql_queued) */
  43        unsigned int    num_queued;             /* Total ever queued */
  44        unsigned int    adj_limit;              /* limit + num_completed */
  45        unsigned int    last_obj_cnt;           /* Count at last queuing */
  46
  47        /* Fields accessed only by completion path (dql_completed) */
  48
  49        unsigned int    limit ____cacheline_aligned_in_smp; /* Current limit */
  50        unsigned int    num_completed;          /* Total ever completed */
  51
  52        unsigned int    prev_ovlimit;           /* Previous over limit */
  53        unsigned int    prev_num_queued;        /* Previous queue total */
  54        unsigned int    prev_last_obj_cnt;      /* Previous queuing cnt */
  55
  56        unsigned int    lowest_slack;           /* Lowest slack found */
  57        unsigned long   slack_start_time;       /* Time slacks seen */
  58
  59        /* Configuration */
  60        unsigned int    max_limit;              /* Max limit */
  61        unsigned int    min_limit;              /* Minimum limit */
  62        unsigned int    slack_hold_time;        /* Time to measure slack */
  63};
  64
  65/* Set some static maximums */
  66#define DQL_MAX_OBJECT (UINT_MAX / 16)
  67#define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT)
  68
  69/*
  70 * Record number of objects queued. Assumes that caller has already checked
  71 * availability in the queue with dql_avail.
  72 */
  73static inline void dql_queued(struct dql *dql, unsigned int count)
  74{
  75        BUG_ON(count > DQL_MAX_OBJECT);
  76
  77        dql->last_obj_cnt = count;
  78
  79        /* We want to force a write first, so that cpu do not attempt
  80         * to get cache line containing last_obj_cnt, num_queued, adj_limit
  81         * in Shared state, but directly does a Request For Ownership
  82         * It is only a hint, we use barrier() only.
  83         */
  84        barrier();
  85
  86        dql->num_queued += count;
  87}
  88
  89/* Returns how many objects can be queued, < 0 indicates over limit. */
  90static inline int dql_avail(const struct dql *dql)
  91{
  92        return READ_ONCE(dql->adj_limit) - READ_ONCE(dql->num_queued);
  93}
  94
  95/* Record number of completed objects and recalculate the limit. */
  96void dql_completed(struct dql *dql, unsigned int count);
  97
  98/* Reset dql state */
  99void dql_reset(struct dql *dql);
 100
 101/* Initialize dql state */
 102void dql_init(struct dql *dql, unsigned int hold_time);
 103
 104#endif /* _KERNEL_ */
 105
 106#endif /* _LINUX_DQL_H */
 107