Root/lib/dynamic_queue_limits.c

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/jiffies.h>
11#include <linux/dynamic_queue_limits.h>
12
13#define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
14#define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)
15
16/* Records completed count and recalculates the queue limit */
17void dql_completed(struct dql *dql, unsigned int count)
18{
19    unsigned int inprogress, prev_inprogress, limit;
20    unsigned int ovlimit, completed, num_queued;
21    bool all_prev_completed;
22
23    num_queued = ACCESS_ONCE(dql->num_queued);
24
25    /* Can't complete more than what's in queue */
26    BUG_ON(count > num_queued - dql->num_completed);
27
28    completed = dql->num_completed + count;
29    limit = dql->limit;
30    ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
31    inprogress = num_queued - completed;
32    prev_inprogress = dql->prev_num_queued - dql->num_completed;
33    all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
34
35    if ((ovlimit && !inprogress) ||
36        (dql->prev_ovlimit && all_prev_completed)) {
37        /*
38         * Queue considered starved if:
39         * - The queue was over-limit in the last interval,
40         * and there is no more data in the queue.
41         * OR
42         * - The queue was over-limit in the previous interval and
43         * when enqueuing it was possible that all queued data
44         * had been consumed. This covers the case when queue
45         * may have becomes starved between completion processing
46         * running and next time enqueue was scheduled.
47         *
48         * When queue is starved increase the limit by the amount
49         * of bytes both sent and completed in the last interval,
50         * plus any previous over-limit.
51         */
52        limit += POSDIFF(completed, dql->prev_num_queued) +
53             dql->prev_ovlimit;
54        dql->slack_start_time = jiffies;
55        dql->lowest_slack = UINT_MAX;
56    } else if (inprogress && prev_inprogress && !all_prev_completed) {
57        /*
58         * Queue was not starved, check if the limit can be decreased.
59         * A decrease is only considered if the queue has been busy in
60         * the whole interval (the check above).
61         *
62         * If there is slack, the amount of execess data queued above
63         * the the amount needed to prevent starvation, the queue limit
64         * can be decreased. To avoid hysteresis we consider the
65         * minimum amount of slack found over several iterations of the
66         * completion routine.
67         */
68        unsigned int slack, slack_last_objs;
69
70        /*
71         * Slack is the maximum of
72         * - The queue limit plus previous over-limit minus twice
73         * the number of objects completed. Note that two times
74         * number of completed bytes is a basis for an upper bound
75         * of the limit.
76         * - Portion of objects in the last queuing operation that
77         * was not part of non-zero previous over-limit. That is
78         * "round down" by non-overlimit portion of the last
79         * queueing operation.
80         */
81        slack = POSDIFF(limit + dql->prev_ovlimit,
82            2 * (completed - dql->num_completed));
83        slack_last_objs = dql->prev_ovlimit ?
84            POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
85
86        slack = max(slack, slack_last_objs);
87
88        if (slack < dql->lowest_slack)
89            dql->lowest_slack = slack;
90
91        if (time_after(jiffies,
92                   dql->slack_start_time + dql->slack_hold_time)) {
93            limit = POSDIFF(limit, dql->lowest_slack);
94            dql->slack_start_time = jiffies;
95            dql->lowest_slack = UINT_MAX;
96        }
97    }
98
99    /* Enforce bounds on limit */
100    limit = clamp(limit, dql->min_limit, dql->max_limit);
101
102    if (limit != dql->limit) {
103        dql->limit = limit;
104        ovlimit = 0;
105    }
106
107    dql->adj_limit = limit + completed;
108    dql->prev_ovlimit = ovlimit;
109    dql->prev_last_obj_cnt = dql->last_obj_cnt;
110    dql->num_completed = completed;
111    dql->prev_num_queued = num_queued;
112}
113EXPORT_SYMBOL(dql_completed);
114
115void dql_reset(struct dql *dql)
116{
117    /* Reset all dynamic values */
118    dql->limit = 0;
119    dql->num_queued = 0;
120    dql->num_completed = 0;
121    dql->last_obj_cnt = 0;
122    dql->prev_num_queued = 0;
123    dql->prev_last_obj_cnt = 0;
124    dql->prev_ovlimit = 0;
125    dql->lowest_slack = UINT_MAX;
126    dql->slack_start_time = jiffies;
127}
128EXPORT_SYMBOL(dql_reset);
129
130int dql_init(struct dql *dql, unsigned hold_time)
131{
132    dql->max_limit = DQL_MAX_LIMIT;
133    dql->min_limit = 0;
134    dql->slack_hold_time = hold_time;
135    dql_reset(dql);
136    return 0;
137}
138EXPORT_SYMBOL(dql_init);
139

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