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