| 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); |