1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * Dynamic byte queue limits. See include/linux/dynamic_queue_limits.h
4 *
5 * Copyright (c) 2011, Tom Herbert <therbert@google.com>
6 */
7 #include <linux/types.h>
8 #include <linux/kernel.h>
9 #include <linux/jiffies.h>
10 #include <linux/dynamic_queue_limits.h>
11 #include <linux/compiler.h>
12 #include <linux/export.h>
13 #include <trace/events/napi.h>
14
15 #define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
16 #define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)
17
dql_check_stall(struct dql * dql)18 static void dql_check_stall(struct dql *dql)
19 {
20 unsigned short stall_thrs;
21 unsigned long now;
22
23 stall_thrs = READ_ONCE(dql->stall_thrs);
24 if (!stall_thrs)
25 return;
26
27 now = jiffies;
28 /* Check for a potential stall */
29 if (time_after_eq(now, dql->last_reap + stall_thrs)) {
30 unsigned long hist_head, t, start, end;
31
32 /* We are trying to detect a period of at least @stall_thrs
33 * jiffies without any Tx completions, but during first half
34 * of which some Tx was posted.
35 */
36 dqs_again:
37 hist_head = READ_ONCE(dql->history_head);
38 /* pairs with smp_wmb() in dql_queued() */
39 smp_rmb();
40
41 /* Get the previous entry in the ring buffer, which is the
42 * oldest sample.
43 */
44 start = (hist_head - DQL_HIST_LEN + 1) * BITS_PER_LONG;
45
46 /* Advance start to continue from the last reap time */
47 if (time_before(start, dql->last_reap + 1))
48 start = dql->last_reap + 1;
49
50 /* Newest sample we should have already seen a completion for */
51 end = hist_head * BITS_PER_LONG + (BITS_PER_LONG - 1);
52
53 /* Shrink the search space to [start, (now - start_thrs/2)] if
54 * `end` is beyond the stall zone
55 */
56 if (time_before(now, end + stall_thrs / 2))
57 end = now - stall_thrs / 2;
58
59 /* Search for the queued time in [t, end] */
60 for (t = start; time_before_eq(t, end); t++)
61 if (test_bit(t % (DQL_HIST_LEN * BITS_PER_LONG),
62 dql->history))
63 break;
64
65 /* Variable t contains the time of the queue */
66 if (!time_before_eq(t, end))
67 goto no_stall;
68
69 /* The ring buffer was modified in the meantime, retry */
70 if (hist_head != READ_ONCE(dql->history_head))
71 goto dqs_again;
72
73 dql->stall_cnt++;
74 dql->stall_max = max_t(unsigned short, dql->stall_max, now - t);
75
76 trace_dql_stall_detected(dql->stall_thrs, now - t,
77 dql->last_reap, dql->history_head,
78 now, dql->history);
79 }
80 no_stall:
81 dql->last_reap = now;
82 }
83
84 /* Records completed count and recalculates the queue limit */
dql_completed(struct dql * dql,unsigned int count)85 void dql_completed(struct dql *dql, unsigned int count)
86 {
87 unsigned int inprogress, prev_inprogress, limit;
88 unsigned int ovlimit, completed, num_queued;
89 bool all_prev_completed;
90
91 num_queued = READ_ONCE(dql->num_queued);
92
93 /* Can't complete more than what's in queue */
94 BUG_ON(count > num_queued - dql->num_completed);
95
96 completed = dql->num_completed + count;
97 limit = dql->limit;
98 ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
99 inprogress = num_queued - completed;
100 prev_inprogress = dql->prev_num_queued - dql->num_completed;
101 all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
102
103 if ((ovlimit && !inprogress) ||
104 (dql->prev_ovlimit && all_prev_completed)) {
105 /*
106 * Queue considered starved if:
107 * - The queue was over-limit in the last interval,
108 * and there is no more data in the queue.
109 * OR
110 * - The queue was over-limit in the previous interval and
111 * when enqueuing it was possible that all queued data
112 * had been consumed. This covers the case when queue
113 * may have becomes starved between completion processing
114 * running and next time enqueue was scheduled.
115 *
116 * When queue is starved increase the limit by the amount
117 * of bytes both sent and completed in the last interval,
118 * plus any previous over-limit.
119 */
120 limit += POSDIFF(completed, dql->prev_num_queued) +
121 dql->prev_ovlimit;
122 dql->slack_start_time = jiffies;
123 dql->lowest_slack = UINT_MAX;
124 } else if (inprogress && prev_inprogress && !all_prev_completed) {
125 /*
126 * Queue was not starved, check if the limit can be decreased.
127 * A decrease is only considered if the queue has been busy in
128 * the whole interval (the check above).
129 *
130 * If there is slack, the amount of excess data queued above
131 * the amount needed to prevent starvation, the queue limit
132 * can be decreased. To avoid hysteresis we consider the
133 * minimum amount of slack found over several iterations of the
134 * completion routine.
135 */
136 unsigned int slack, slack_last_objs;
137
138 /*
139 * Slack is the maximum of
140 * - The queue limit plus previous over-limit minus twice
141 * the number of objects completed. Note that two times
142 * number of completed bytes is a basis for an upper bound
143 * of the limit.
144 * - Portion of objects in the last queuing operation that
145 * was not part of non-zero previous over-limit. That is
146 * "round down" by non-overlimit portion of the last
147 * queueing operation.
148 */
149 slack = POSDIFF(limit + dql->prev_ovlimit,
150 2 * (completed - dql->num_completed));
151 slack_last_objs = dql->prev_ovlimit ?
152 POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
153
154 slack = max(slack, slack_last_objs);
155
156 if (slack < dql->lowest_slack)
157 dql->lowest_slack = slack;
158
159 if (time_after(jiffies,
160 dql->slack_start_time + dql->slack_hold_time)) {
161 limit = POSDIFF(limit, dql->lowest_slack);
162 dql->slack_start_time = jiffies;
163 dql->lowest_slack = UINT_MAX;
164 }
165 }
166
167 /* Enforce bounds on limit */
168 limit = clamp(limit, dql->min_limit, dql->max_limit);
169
170 if (limit != dql->limit) {
171 dql->limit = limit;
172 ovlimit = 0;
173 }
174
175 dql->adj_limit = limit + completed;
176 dql->prev_ovlimit = ovlimit;
177 dql->prev_last_obj_cnt = dql->last_obj_cnt;
178 dql->num_completed = completed;
179 dql->prev_num_queued = num_queued;
180
181 dql_check_stall(dql);
182 }
183 EXPORT_SYMBOL(dql_completed);
184
dql_reset(struct dql * dql)185 void dql_reset(struct dql *dql)
186 {
187 /* Reset all dynamic values */
188 dql->limit = 0;
189 dql->num_queued = 0;
190 dql->num_completed = 0;
191 dql->last_obj_cnt = 0;
192 dql->prev_num_queued = 0;
193 dql->prev_last_obj_cnt = 0;
194 dql->prev_ovlimit = 0;
195 dql->lowest_slack = UINT_MAX;
196 dql->slack_start_time = jiffies;
197
198 dql->last_reap = jiffies;
199 dql->history_head = jiffies / BITS_PER_LONG;
200 memset(dql->history, 0, sizeof(dql->history));
201 }
202 EXPORT_SYMBOL(dql_reset);
203
dql_init(struct dql * dql,unsigned int hold_time)204 void dql_init(struct dql *dql, unsigned int hold_time)
205 {
206 dql->max_limit = DQL_MAX_LIMIT;
207 dql->min_limit = 0;
208 dql->slack_hold_time = hold_time;
209 dql->stall_thrs = 0;
210 dql_reset(dql);
211 }
212 EXPORT_SYMBOL(dql_init);
213