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 | */ |
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. */ |
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 | |