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