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 */
dql_completed(struct dql * dql,unsigned int count)17 void 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 }
113 EXPORT_SYMBOL(dql_completed);
114
dql_reset(struct dql * dql)115 void 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 }
128 EXPORT_SYMBOL(dql_reset);
129
dql_init(struct dql * dql,unsigned hold_time)130 int 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 }
138 EXPORT_SYMBOL(dql_init);
139