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 14 #define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0) 15 #define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0) 16 17 /* Records completed count and recalculates the queue limit */ 18 void dql_completed(struct dql *dql, unsigned int count) 19 { 20 unsigned int inprogress, prev_inprogress, limit; 21 unsigned int ovlimit, completed, num_queued; 22 bool all_prev_completed; 23 24 num_queued = READ_ONCE(dql->num_queued); 25 26 /* Can't complete more than what's in queue */ 27 BUG_ON(count > num_queued - dql->num_completed); 28 29 completed = dql->num_completed + count; 30 limit = dql->limit; 31 ovlimit = POSDIFF(num_queued - dql->num_completed, limit); 32 inprogress = num_queued - completed; 33 prev_inprogress = dql->prev_num_queued - dql->num_completed; 34 all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued); 35 36 if ((ovlimit && !inprogress) || 37 (dql->prev_ovlimit && all_prev_completed)) { 38 /* 39 * Queue considered starved if: 40 * - The queue was over-limit in the last interval, 41 * and there is no more data in the queue. 42 * OR 43 * - The queue was over-limit in the previous interval and 44 * when enqueuing it was possible that all queued data 45 * had been consumed. This covers the case when queue 46 * may have becomes starved between completion processing 47 * running and next time enqueue was scheduled. 48 * 49 * When queue is starved increase the limit by the amount 50 * of bytes both sent and completed in the last interval, 51 * plus any previous over-limit. 52 */ 53 limit += POSDIFF(completed, dql->prev_num_queued) + 54 dql->prev_ovlimit; 55 dql->slack_start_time = jiffies; 56 dql->lowest_slack = UINT_MAX; 57 } else if (inprogress && prev_inprogress && !all_prev_completed) { 58 /* 59 * Queue was not starved, check if the limit can be decreased. 60 * A decrease is only considered if the queue has been busy in 61 * the whole interval (the check above). 62 * 63 * If there is slack, the amount of excess data queued above 64 * the amount needed to prevent starvation, the queue limit 65 * can be decreased. To avoid hysteresis we consider the 66 * minimum amount of slack found over several iterations of the 67 * completion routine. 68 */ 69 unsigned int slack, slack_last_objs; 70 71 /* 72 * Slack is the maximum of 73 * - The queue limit plus previous over-limit minus twice 74 * the number of objects completed. Note that two times 75 * number of completed bytes is a basis for an upper bound 76 * of the limit. 77 * - Portion of objects in the last queuing operation that 78 * was not part of non-zero previous over-limit. That is 79 * "round down" by non-overlimit portion of the last 80 * queueing operation. 81 */ 82 slack = POSDIFF(limit + dql->prev_ovlimit, 83 2 * (completed - dql->num_completed)); 84 slack_last_objs = dql->prev_ovlimit ? 85 POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0; 86 87 slack = max(slack, slack_last_objs); 88 89 if (slack < dql->lowest_slack) 90 dql->lowest_slack = slack; 91 92 if (time_after(jiffies, 93 dql->slack_start_time + dql->slack_hold_time)) { 94 limit = POSDIFF(limit, dql->lowest_slack); 95 dql->slack_start_time = jiffies; 96 dql->lowest_slack = UINT_MAX; 97 } 98 } 99 100 /* Enforce bounds on limit */ 101 limit = clamp(limit, dql->min_limit, dql->max_limit); 102 103 if (limit != dql->limit) { 104 dql->limit = limit; 105 ovlimit = 0; 106 } 107 108 dql->adj_limit = limit + completed; 109 dql->prev_ovlimit = ovlimit; 110 dql->prev_last_obj_cnt = dql->last_obj_cnt; 111 dql->num_completed = completed; 112 dql->prev_num_queued = num_queued; 113 } 114 EXPORT_SYMBOL(dql_completed); 115 116 void dql_reset(struct dql *dql) 117 { 118 /* Reset all dynamic values */ 119 dql->limit = 0; 120 dql->num_queued = 0; 121 dql->num_completed = 0; 122 dql->last_obj_cnt = 0; 123 dql->prev_num_queued = 0; 124 dql->prev_last_obj_cnt = 0; 125 dql->prev_ovlimit = 0; 126 dql->lowest_slack = UINT_MAX; 127 dql->slack_start_time = jiffies; 128 } 129 EXPORT_SYMBOL(dql_reset); 130 131 void dql_init(struct dql *dql, unsigned int hold_time) 132 { 133 dql->max_limit = DQL_MAX_LIMIT; 134 dql->min_limit = 0; 135 dql->slack_hold_time = hold_time; 136 dql_reset(dql); 137 } 138 EXPORT_SYMBOL(dql_init); 139