include/linux/dynamic_queue_limits.h

   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
  40 struct 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  */
  72 static inline void dql_queued(struct dql *dql, unsigned int count)
  73 {
  74         BUG_ON(count > DQL_MAX_OBJECT);
  75
  76         dql->last_obj_cnt = count;
  77
  78         /* We want to force a write first, so that cpu do not attempt
  79          * to get cache line containing last_obj_cnt, num_queued, adj_limit
  80          * in Shared state, but directly does a Request For Ownership
  81          * It is only a hint, we use barrier() only.
  82          */
  83         barrier();
  84
  85         dql->num_queued += count;
  86 }
  87
  88 /* Returns how many objects can be queued, < 0 indicates over limit. */
  89 static inline int dql_avail(const struct dql *dql)
  90 {
  91         return ACCESS_ONCE(dql->adj_limit) - ACCESS_ONCE(dql->num_queued);
  92 }
  93
  94 /* Record number of completed objects and recalculate the limit. */
  95 void dql_completed(struct dql *dql, unsigned int count);
  96
  97 /* Reset dql state */
  98 void dql_reset(struct dql *dql);
  99
 100 /* Initialize dql state */
 101 int dql_init(struct dql *dql, unsigned hold_time);
 102
 103 #endif /* _KERNEL_ */
 104
 105 #endif /* _LINUX_DQL_H */