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75957ba3 TH |
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 | ||
75957ba3 | 76 | dql->last_obj_cnt = count; |
3d9a0d2f ED |
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; | |
75957ba3 TH |
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 | { | |
3d9a0d2f | 91 | return ACCESS_ONCE(dql->adj_limit) - ACCESS_ONCE(dql->num_queued); |
75957ba3 TH |
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 */ |