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