1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * Dynamic byte queue limits. See include/linux/dynamic_queue_limits.h
4 *
5 * Copyright (c) 2011, Tom Herbert <therbert@google.com>
6 */
7 #include <linux/types.h>
8 #include <linux/kernel.h>
9 #include <linux/jiffies.h>
10 #include <linux/dynamic_queue_limits.h>
11 #include <linux/compiler.h>
12 #include <linux/export.h>
13
14 #define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
15 #define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)
16
17 /* Records completed count and recalculates the queue limit */
18 void dql_completed(struct dql *dql, unsigned int count)
19 {
20 unsigned int inprogress, prev_inprogress, limit;
21 unsigned int ovlimit, completed, num_queued;
22 bool all_prev_completed;
23
24 num_queued = READ_ONCE(dql->num_queued);
25
26 /* Can't complete more than what's in queue */
27 BUG_ON(count > num_queued - dql->num_completed);
28
29 completed = dql->num_completed + count;
30 limit = dql->limit;
31 ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
32 inprogress = num_queued - completed;
33 prev_inprogress = dql->prev_num_queued - dql->num_completed;
34 all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
35
36 if ((ovlimit && !inprogress) ||
37 (dql->prev_ovlimit && all_prev_completed)) {
38 /*
39 * Queue considered starved if:
40 * - The queue was over-limit in the last interval,
41 * and there is no more data in the queue.
42 * OR
43 * - The queue was over-limit in the previous interval and
44 * when enqueuing it was possible that all queued data
45 * had been consumed. This covers the case when queue
46 * may have becomes starved between completion processing
47 * running and next time enqueue was scheduled.
48 *
49 * When queue is starved increase the limit by the amount
50 * of bytes both sent and completed in the last interval,
51 * plus any previous over-limit.
52 */
53 limit += POSDIFF(completed, dql->prev_num_queued) +
54 dql->prev_ovlimit;
55 dql->slack_start_time = jiffies;
56 dql->lowest_slack = UINT_MAX;
57 } else if (inprogress && prev_inprogress && !all_prev_completed) {
58 /*
59 * Queue was not starved, check if the limit can be decreased.
60 * A decrease is only considered if the queue has been busy in
61 * the whole interval (the check above).
62 *
63 * If there is slack, the amount of execess data queued above
64 * the the amount needed to prevent starvation, the queue limit
65 * can be decreased. To avoid hysteresis we consider the
66 * minimum amount of slack found over several iterations of the
67 * completion routine.
68 */
69 unsigned int slack, slack_last_objs;
70
71 /*
72 * Slack is the maximum of
73 * - The queue limit plus previous over-limit minus twice
74 * the number of objects completed. Note that two times
75 * number of completed bytes is a basis for an upper bound
76 * of the limit.
77 * - Portion of objects in the last queuing operation that
78 * was not part of non-zero previous over-limit. That is
79 * "round down" by non-overlimit portion of the last
80 * queueing operation.
81 */
82 slack = POSDIFF(limit + dql->prev_ovlimit,
83 2 * (completed - dql->num_completed));
84 slack_last_objs = dql->prev_ovlimit ?
85 POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
86
87 slack = max(slack, slack_last_objs);
88
89 if (slack < dql->lowest_slack)
90 dql->lowest_slack = slack;
91
92 if (time_after(jiffies,
93 dql->slack_start_time + dql->slack_hold_time)) {
94 limit = POSDIFF(limit, dql->lowest_slack);
95 dql->slack_start_time = jiffies;
96 dql->lowest_slack = UINT_MAX;
97 }
98 }
99
100 /* Enforce bounds on limit */
101 limit = clamp(limit, dql->min_limit, dql->max_limit);
102
103 if (limit != dql->limit) {
104 dql->limit = limit;
105 ovlimit = 0;
106 }
107
108 dql->adj_limit = limit + completed;
109 dql->prev_ovlimit = ovlimit;
110 dql->prev_last_obj_cnt = dql->last_obj_cnt;
111 dql->num_completed = completed;
112 dql->prev_num_queued = num_queued;
113 }
114 EXPORT_SYMBOL(dql_completed);
115
116 void dql_reset(struct dql *dql)
117 {
118 /* Reset all dynamic values */
119 dql->limit = 0;
120 dql->num_queued = 0;
121 dql->num_completed = 0;
122 dql->last_obj_cnt = 0;
123 dql->prev_num_queued = 0;
124 dql->prev_last_obj_cnt = 0;
125 dql->prev_ovlimit = 0;
126 dql->lowest_slack = UINT_MAX;
127 dql->slack_start_time = jiffies;
128 }
129 EXPORT_SYMBOL(dql_reset);
130
131 void dql_init(struct dql *dql, unsigned int hold_time)
132 {
133 dql->max_limit = DQL_MAX_LIMIT;
134 dql->min_limit = 0;
135 dql->slack_hold_time = hold_time;
136 dql_reset(dql);
137 }
138 EXPORT_SYMBOL(dql_init);