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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  #include <asm/bug.h>
42  
43  struct dql {
44  	/* Fields accessed in enqueue path (dql_queued) */
45  	unsigned int	num_queued;		/* Total ever queued */
46  	unsigned int	adj_limit;		/* limit + num_completed */
47  	unsigned int	last_obj_cnt;		/* Count at last queuing */
48  
49  	/* Fields accessed only by completion path (dql_completed) */
50  
51  	unsigned int	limit ____cacheline_aligned_in_smp; /* Current limit */
52  	unsigned int	num_completed;		/* Total ever completed */
53  
54  	unsigned int	prev_ovlimit;		/* Previous over limit */
55  	unsigned int	prev_num_queued;	/* Previous queue total */
56  	unsigned int	prev_last_obj_cnt;	/* Previous queuing cnt */
57  
58  	unsigned int	lowest_slack;		/* Lowest slack found */
59  	unsigned long	slack_start_time;	/* Time slacks seen */
60  
61  	/* Configuration */
62  	unsigned int	max_limit;		/* Max limit */
63  	unsigned int	min_limit;		/* Minimum limit */
64  	unsigned int	slack_hold_time;	/* Time to measure slack */
65  };
66  
67  /* Set some static maximums */
68  #define DQL_MAX_OBJECT (UINT_MAX / 16)
69  #define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT)
70  
71  /*
72   * Record number of objects queued. Assumes that caller has already checked
73   * availability in the queue with dql_avail.
74   */
dql_queued(struct dql * dql,unsigned int count)75  static inline void dql_queued(struct dql *dql, unsigned int count)
76  {
77  	BUG_ON(count > DQL_MAX_OBJECT);
78  
79  	dql->last_obj_cnt = count;
80  
81  	/* We want to force a write first, so that cpu do not attempt
82  	 * to get cache line containing last_obj_cnt, num_queued, adj_limit
83  	 * in Shared state, but directly does a Request For Ownership
84  	 * It is only a hint, we use barrier() only.
85  	 */
86  	barrier();
87  
88  	dql->num_queued += count;
89  }
90  
91  /* Returns how many objects can be queued, < 0 indicates over limit. */
dql_avail(const struct dql * dql)92  static inline int dql_avail(const struct dql *dql)
93  {
94  	return READ_ONCE(dql->adj_limit) - READ_ONCE(dql->num_queued);
95  }
96  
97  /* Record number of completed objects and recalculate the limit. */
98  void dql_completed(struct dql *dql, unsigned int count);
99  
100  /* Reset dql state */
101  void dql_reset(struct dql *dql);
102  
103  /* Initialize dql state */
104  void dql_init(struct dql *dql, unsigned int hold_time);
105  
106  #endif /* _KERNEL_ */
107  
108  #endif /* _LINUX_DQL_H */
109