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
18 static void dql_check_stall(struct dql *dql, unsigned short stall_thrs)
19 {
20 unsigned long now;
21
22 if (!stall_thrs)
23 return;
24
25 now = jiffies;
26 /* Check for a potential stall */
27 if (time_after_eq(now, dql->last_reap + stall_thrs)) {
28 unsigned long hist_head, t, start, end;
29
30 /* We are trying to detect a period of at least @stall_thrs
31 * jiffies without any Tx completions, but during first half
32 * of which some Tx was posted.
33 */
34 dqs_again:
35 hist_head = READ_ONCE(dql->history_head);
36 /* pairs with smp_wmb() in dql_queued() */
37 smp_rmb();
38
39 /* Get the previous entry in the ring buffer, which is the
40 * oldest sample.
41 */
42 start = (hist_head - DQL_HIST_LEN + 1) * BITS_PER_LONG;
43
44 /* Advance start to continue from the last reap time */
45 if (time_before(start, dql->last_reap + 1))
46 start = dql->last_reap + 1;
47
48 /* Newest sample we should have already seen a completion for */
49 end = hist_head * BITS_PER_LONG + (BITS_PER_LONG - 1);
50
51 /* Shrink the search space to [start, (now - start_thrs/2)] if
52 * `end` is beyond the stall zone
53 */
54 if (time_before(now, end + stall_thrs / 2))
55 end = now - stall_thrs / 2;
56
57 /* Search for the queued time in [t, end] */
58 for (t = start; time_before_eq(t, end); t++)
59 if (test_bit(t % (DQL_HIST_LEN * BITS_PER_LONG),
60 dql->history))
61 break;
62
63 /* Variable t contains the time of the queue */
64 if (!time_before_eq(t, end))
65 goto no_stall;
66
67 /* The ring buffer was modified in the meantime, retry */
68 if (hist_head != READ_ONCE(dql->history_head))
69 goto dqs_again;
70
71 dql->stall_cnt++;
72 dql->stall_max = max_t(unsigned short, dql->stall_max, now - t);
73
74 trace_dql_stall_detected(dql->stall_thrs, now - t,
75 dql->last_reap, dql->history_head,
76 now, dql->history);
77 }
78 no_stall:
79 dql->last_reap = now;
80 }
81
82 /* Records completed count and recalculates the queue limit */
83 void dql_completed(struct dql *dql, unsigned int count)
84 {
85 unsigned int inprogress, prev_inprogress, limit;
86 unsigned int ovlimit, completed, num_queued;
87 unsigned short stall_thrs;
88 bool all_prev_completed;
89
90 num_queued = READ_ONCE(dql->num_queued);
91 /* Read stall_thrs in advance since it belongs to the same (first)
92 * cache line as ->num_queued. This way, dql_check_stall() does not
93 * need to touch the first cache line again later, reducing the window
94 * of possible false sharing.
95 */
96 stall_thrs = READ_ONCE(dql->stall_thrs);
97
98 /* Can't complete more than what's in queue */
99 BUG_ON(count > num_queued - dql->num_completed);
100
101 completed = dql->num_completed + count;
102 limit = dql->limit;
103 ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
104 inprogress = num_queued - completed;
105 prev_inprogress = dql->prev_num_queued - dql->num_completed;
106 all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
107
108 if ((ovlimit && !inprogress) ||
109 (dql->prev_ovlimit && all_prev_completed)) {
110 /*
111 * Queue considered starved if:
112 * - The queue was over-limit in the last interval,
113 * and there is no more data in the queue.
114 * OR
115 * - The queue was over-limit in the previous interval and
116 * when enqueuing it was possible that all queued data
117 * had been consumed. This covers the case when queue
118 * may have becomes starved between completion processing
119 * running and next time enqueue was scheduled.
120 *
121 * When queue is starved increase the limit by the amount
122 * of bytes both sent and completed in the last interval,
123 * plus any previous over-limit.
124 */
125 limit += POSDIFF(completed, dql->prev_num_queued) +
126 dql->prev_ovlimit;
127 dql->slack_start_time = jiffies;
128 dql->lowest_slack = UINT_MAX;
129 } else if (inprogress && prev_inprogress && !all_prev_completed) {
130 /*
131 * Queue was not starved, check if the limit can be decreased.
132 * A decrease is only considered if the queue has been busy in
133 * the whole interval (the check above).
134 *
135 * If there is slack, the amount of excess data queued above
136 * the amount needed to prevent starvation, the queue limit
137 * can be decreased. To avoid hysteresis we consider the
138 * minimum amount of slack found over several iterations of the
139 * completion routine.
140 */
141 unsigned int slack, slack_last_objs;
142
143 /*
144 * Slack is the maximum of
145 * - The queue limit plus previous over-limit minus twice
146 * the number of objects completed. Note that two times
147 * number of completed bytes is a basis for an upper bound
148 * of the limit.
149 * - Portion of objects in the last queuing operation that
150 * was not part of non-zero previous over-limit. That is
151 * "round down" by non-overlimit portion of the last
152 * queueing operation.
153 */
154 slack = POSDIFF(limit + dql->prev_ovlimit,
155 2 * (completed - dql->num_completed));
156 slack_last_objs = dql->prev_ovlimit ?
157 POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
158
159 slack = max(slack, slack_last_objs);
160
161 if (slack < dql->lowest_slack)
162 dql->lowest_slack = slack;
163
164 if (time_after(jiffies,
165 dql->slack_start_time + dql->slack_hold_time)) {
166 limit = POSDIFF(limit, dql->lowest_slack);
167 dql->slack_start_time = jiffies;
168 dql->lowest_slack = UINT_MAX;
169 }
170 }
171
172 /* Enforce bounds on limit */
173 limit = clamp(limit, dql->min_limit, dql->max_limit);
174
175 if (limit != dql->limit) {
176 dql->limit = limit;
177 ovlimit = 0;
178 }
179
180 dql->adj_limit = limit + completed;
181 dql->prev_ovlimit = ovlimit;
182 dql->prev_last_obj_cnt = dql->last_obj_cnt;
183 dql->num_completed = completed;
184 dql->prev_num_queued = num_queued;
185
186 dql_check_stall(dql, stall_thrs);
187 }
188 EXPORT_SYMBOL(dql_completed);
189
190 void dql_reset(struct dql *dql)
191 {
192 /* Reset all dynamic values */
193 dql->limit = 0;
194 dql->num_queued = 0;
195 dql->num_completed = 0;
196 dql->last_obj_cnt = 0;
197 dql->prev_num_queued = 0;
198 dql->prev_last_obj_cnt = 0;
199 dql->prev_ovlimit = 0;
200 dql->lowest_slack = UINT_MAX;
201 dql->slack_start_time = jiffies;
202
203 dql->last_reap = jiffies;
204 dql->history_head = jiffies / BITS_PER_LONG;
205 memset(dql->history, 0, sizeof(dql->history));
206 }
207 EXPORT_SYMBOL(dql_reset);
208
209 void dql_init(struct dql *dql, unsigned int hold_time)
210 {
211 dql->max_limit = DQL_MAX_LIMIT;
212 dql->min_limit = 0;
213 dql->slack_hold_time = hold_time;
214 dql->stall_thrs = 0;
215 dql_reset(dql);
216 }
217 EXPORT_SYMBOL(dql_init);
218
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