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 <linux/bitops.h>
42 #include <asm/bug.h>
43
44 #define DQL_HIST_LEN 4
45 #define DQL_HIST_ENT(dql, idx) ((dql)->history[(idx) % DQL_HIST_LEN])
46
47 struct dql {
48 /* Fields accessed in enqueue path (dql_queued) */
49 unsigned int num_queued; /* Total ever queued */
50 unsigned int adj_limit; /* limit + num_completed */
51 unsigned int last_obj_cnt; /* Count at last queuing */
52
53 /* Stall threshold (in jiffies), defined by user */
54 unsigned short stall_thrs;
55
56 unsigned long history_head; /* top 58 bits of jiffies */
57 /* stall entries, a bit per entry */
58 unsigned long history[DQL_HIST_LEN];
59
60 /* Fields accessed only by completion path (dql_completed) */
61
62 unsigned int limit ____cacheline_aligned_in_smp; /* Current limit */
63 unsigned int num_completed; /* Total ever completed */
64
65 unsigned int prev_ovlimit; /* Previous over limit */
66 unsigned int prev_num_queued; /* Previous queue total */
67 unsigned int prev_last_obj_cnt; /* Previous queuing cnt */
68
69 unsigned int lowest_slack; /* Lowest slack found */
70 unsigned long slack_start_time; /* Time slacks seen */
71
72 /* Configuration */
73 unsigned int max_limit; /* Max limit */
74 unsigned int min_limit; /* Minimum limit */
75 unsigned int slack_hold_time; /* Time to measure slack */
76
77 /* Longest stall detected, reported to user */
78 unsigned short stall_max;
79 unsigned long last_reap; /* Last reap (in jiffies) */
80 unsigned long stall_cnt; /* Number of stalls */
81 };
82
83 /* Set some static maximums */
84 #define DQL_MAX_OBJECT (UINT_MAX / 16)
85 #define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT)
86
87 /* Populate the bitmap to be processed later in dql_check_stall() */
88 static inline void dql_queue_stall(struct dql *dql)
89 {
90 unsigned long map, now, now_hi, i;
91
92 now = jiffies;
93 now_hi = now / BITS_PER_LONG;
94
95 /* The following code set a bit in the ring buffer, where each
96 * bit trackes time the packet was queued. The dql->history buffer
97 * tracks DQL_HIST_LEN * BITS_PER_LONG time (jiffies) slot
98 */
99 if (unlikely(now_hi != dql->history_head)) {
100 /* About to reuse slots, clear them */
101 for (i = 0; i < DQL_HIST_LEN; i++) {
102 /* Multiplication masks high bits */
103 if (now_hi * BITS_PER_LONG ==
104 (dql->history_head + i) * BITS_PER_LONG)
105 break;
106 DQL_HIST_ENT(dql, dql->history_head + i + 1) = 0;
107 }
108 /* pairs with smp_rmb() in dql_check_stall() */
109 smp_wmb();
110 WRITE_ONCE(dql->history_head, now_hi);
111 }
112
113 /* __set_bit() does not guarantee WRITE_ONCE() semantics */
114 map = DQL_HIST_ENT(dql, now_hi);
115
116 /* Populate the history with an entry (bit) per queued */
117 if (!(map & BIT_MASK(now)))
118 WRITE_ONCE(DQL_HIST_ENT(dql, now_hi), map | BIT_MASK(now));
119 }
120
121 /*
122 * Record number of objects queued. Assumes that caller has already checked
123 * availability in the queue with dql_avail.
124 */
125 static inline void dql_queued(struct dql *dql, unsigned int count)
126 {
127 if (WARN_ON_ONCE(count > DQL_MAX_OBJECT))
128 return;
129
130 dql->last_obj_cnt = count;
131
132 /* We want to force a write first, so that cpu do not attempt
133 * to get cache line containing last_obj_cnt, num_queued, adj_limit
134 * in Shared state, but directly does a Request For Ownership
135 * It is only a hint, we use barrier() only.
136 */
137 barrier();
138
139 dql->num_queued += count;
140
141 /* Only populate stall information if the threshold is set */
142 if (READ_ONCE(dql->stall_thrs))
143 dql_queue_stall(dql);
144 }
145
146 /* Returns how many objects can be queued, < 0 indicates over limit. */
147 static inline int dql_avail(const struct dql *dql)
148 {
149 return READ_ONCE(dql->adj_limit) - READ_ONCE(dql->num_queued);
150 }
151
152 /* Record number of completed objects and recalculate the limit. */
153 void dql_completed(struct dql *dql, unsigned int count);
154
155 /* Reset dql state */
156 void dql_reset(struct dql *dql);
157
158 /* Initialize dql state */
159 void dql_init(struct dql *dql, unsigned int hold_time);
160
161 #endif /* _KERNEL_ */
162
163 #endif /* _LINUX_DQL_H */
164
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