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 */