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 */
dql_queued(struct dql * dql,unsigned int count)72 static inline void dql_queued(struct dql *dql, unsigned int count)
73 {
74 BUG_ON(count > DQL_MAX_OBJECT);
75
76 dql->num_queued += count;
77 dql->last_obj_cnt = count;
78 }
79
80 /* Returns how many objects can be queued, < 0 indicates over limit. */
dql_avail(const struct dql * dql)81 static inline int dql_avail(const struct dql *dql)
82 {
83 return dql->adj_limit - dql->num_queued;
84 }
85
86 /* Record number of completed objects and recalculate the limit. */
87 void dql_completed(struct dql *dql, unsigned int count);
88
89 /* Reset dql state */
90 void dql_reset(struct dql *dql);
91
92 /* Initialize dql state */
93 int dql_init(struct dql *dql, unsigned hold_time);
94
95 #endif /* _KERNEL_ */
96
97 #endif /* _LINUX_DQL_H */
98