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) 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 */ 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); 184 185 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); 203 204 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