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  */
dql_queued(struct dql * dql,unsigned int count)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. */
dql_avail(const struct dql * dql)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 */
107