include/linux/dynamic_queue_limits.h

   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
  41 struct 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  */
  73 static 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. */
  90 static 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. */
  96 void dql_completed(struct dql *dql, unsigned int count);
  97
  98 /* Reset dql state */
  99 void dql_reset(struct dql *dql);
 100
 101 /* Initialize dql state */
 102 void dql_init(struct dql *dql, unsigned int hold_time);
 103
 104 #endif /* _KERNEL_ */
 105
 106 #endif /* _LINUX_DQL_H */