1 /* SPDX-License-Identifier: GPL-2.0 */
3 * Dynamic queue limits (dql) - Definitions
5 * Copyright (c) 2011, Tom Herbert <therbert@google.com>
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:
11 * 1) Objects are queued up to some limit specified as number of objects.
12 * 2) Periodically a completion process executes which retires consumed
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.
19 * The goal of dql is to calculate the limit as the minimum number of objects
20 * needed to prevent starvation.
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
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).
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 */
49 /* Fields accessed only by completion path (dql_completed) */
51 unsigned int limit ____cacheline_aligned_in_smp
; /* Current limit */
52 unsigned int num_completed
; /* Total ever completed */
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 */
58 unsigned int lowest_slack
; /* Lowest slack found */
59 unsigned long slack_start_time
; /* Time slacks seen */
62 unsigned int max_limit
; /* Max limit */
63 unsigned int min_limit
; /* Minimum limit */
64 unsigned int slack_hold_time
; /* Time to measure slack */
67 /* Set some static maximums */
68 #define DQL_MAX_OBJECT (UINT_MAX / 16)
69 #define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT)
72 * Record number of objects queued. Assumes that caller has already checked
73 * availability in the queue with dql_avail.
75 static inline void dql_queued(struct dql
*dql
, unsigned int count
)
77 BUG_ON(count
> DQL_MAX_OBJECT
);
79 dql
->last_obj_cnt
= count
;
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.
88 dql
->num_queued
+= count
;
91 /* Returns how many objects can be queued, < 0 indicates over limit. */
92 static inline int dql_avail(const struct dql
*dql
)
94 return READ_ONCE(dql
->adj_limit
) - READ_ONCE(dql
->num_queued
);
97 /* Record number of completed objects and recalculate the limit. */
98 void dql_completed(struct dql
*dql
, unsigned int count
);
100 /* Reset dql state */
101 void dql_reset(struct dql
*dql
);
103 /* Initialize dql state */
104 void dql_init(struct dql
*dql
, unsigned int hold_time
);
106 #endif /* _KERNEL_ */
108 #endif /* _LINUX_DQL_H */