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/types.h>
7#include <linux/kernel.h>
8#include <linux/jiffies.h>
9#include <linux/dynamic_queue_limits.h>
10#include <linux/compiler.h>
11#include <linux/export.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