0fafa77f403642820578e0b26b9e6bdedcc953a4
[linux-2.6.git] / 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 */
17 void dql_completed(struct dql *dql, unsigned int count)
18 {
19         unsigned int inprogress, prev_inprogress, limit;
20         unsigned int ovlimit, completed;
21         bool all_prev_completed;
22
23         /* Can't complete more than what's in queue */
24         BUG_ON(count > dql->num_queued - dql->num_completed);
25
26         completed = dql->num_completed + count;
27         limit = dql->limit;
28         ovlimit = POSDIFF(dql->num_queued - dql->num_completed, limit);
29         inprogress = dql->num_queued - completed;
30         prev_inprogress = dql->prev_num_queued - dql->num_completed;
31         all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
32
33         if ((ovlimit && !inprogress) ||
34             (dql->prev_ovlimit && all_prev_completed)) {
35                 /*
36                  * Queue considered starved if:
37                  *   - The queue was over-limit in the last interval,
38                  *     and there is no more data in the queue.
39                  *  OR
40                  *   - The queue was over-limit in the previous interval and
41                  *     when enqueuing it was possible that all queued data
42                  *     had been consumed.  This covers the case when queue
43                  *     may have becomes starved between completion processing
44                  *     running and next time enqueue was scheduled.
45                  *
46                  *     When queue is starved increase the limit by the amount
47                  *     of bytes both sent and completed in the last interval,
48                  *     plus any previous over-limit.
49                  */
50                 limit += POSDIFF(completed, dql->prev_num_queued) +
51                      dql->prev_ovlimit;
52                 dql->slack_start_time = jiffies;
53                 dql->lowest_slack = UINT_MAX;
54         } else if (inprogress && prev_inprogress && !all_prev_completed) {
55                 /*
56                  * Queue was not starved, check if the limit can be decreased.
57                  * A decrease is only considered if the queue has been busy in
58                  * the whole interval (the check above).
59                  *
60                  * If there is slack, the amount of execess data queued above
61                  * the the amount needed to prevent starvation, the queue limit
62                  * can be decreased.  To avoid hysteresis we consider the
63                  * minimum amount of slack found over several iterations of the
64                  * completion routine.
65                  */
66                 unsigned int slack, slack_last_objs;
67
68                 /*
69                  * Slack is the maximum of
70                  *   - The queue limit plus previous over-limit minus twice
71                  *     the number of objects completed.  Note that two times
72                  *     number of completed bytes is a basis for an upper bound
73                  *     of the limit.
74                  *   - Portion of objects in the last queuing operation that
75                  *     was not part of non-zero previous over-limit.  That is
76                  *     "round down" by non-overlimit portion of the last
77                  *     queueing operation.
78                  */
79                 slack = POSDIFF(limit + dql->prev_ovlimit,
80                     2 * (completed - dql->num_completed));
81                 slack_last_objs = dql->prev_ovlimit ?
82                     POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
83
84                 slack = max(slack, slack_last_objs);
85
86                 if (slack < dql->lowest_slack)
87                         dql->lowest_slack = slack;
88
89                 if (time_after(jiffies,
90                                dql->slack_start_time + dql->slack_hold_time)) {
91                         limit = POSDIFF(limit, dql->lowest_slack);
92                         dql->slack_start_time = jiffies;
93                         dql->lowest_slack = UINT_MAX;
94                 }
95         }
96
97         /* Enforce bounds on limit */
98         limit = clamp(limit, dql->min_limit, dql->max_limit);
99
100         if (limit != dql->limit) {
101                 dql->limit = limit;
102                 ovlimit = 0;
103         }
104
105         dql->adj_limit = limit + completed;
106         dql->prev_ovlimit = ovlimit;
107         dql->prev_last_obj_cnt = dql->last_obj_cnt;
108         dql->num_completed = completed;
109         dql->prev_num_queued = dql->num_queued;
110 }
111 EXPORT_SYMBOL(dql_completed);
112
113 void dql_reset(struct dql *dql)
114 {
115         /* Reset all dynamic values */
116         dql->limit = 0;
117         dql->num_queued = 0;
118         dql->num_completed = 0;
119         dql->last_obj_cnt = 0;
120         dql->prev_num_queued = 0;
121         dql->prev_last_obj_cnt = 0;
122         dql->prev_ovlimit = 0;
123         dql->lowest_slack = UINT_MAX;
124         dql->slack_start_time = jiffies;
125 }
126 EXPORT_SYMBOL(dql_reset);
127
128 int dql_init(struct dql *dql, unsigned hold_time)
129 {
130         dql->max_limit = DQL_MAX_LIMIT;
131         dql->min_limit = 0;
132         dql->slack_hold_time = hold_time;
133         dql_reset(dql);
134         return 0;
135 }
136 EXPORT_SYMBOL(dql_init);