1 /* 2 * Dynamic byte queue limits. See include/linux/dynamic_queue_limits.h 3 * 4 * Copyright (c) 2011, Tom Herbert <therbert@google.com> 5 */ 6 #include <linux/types.h> 7 #include <linux/kernel.h> 8 #include <linux/jiffies.h> 9 #include <linux/dynamic_queue_limits.h> 10 #include <linux/compiler.h> 11 #include <linux/export.h> 12 13 #define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0) 14 #define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0) 15 16 /* Records completed count and recalculates the queue limit */ 17 void dql_completed(struct dql *dql, unsigned int count) 18 { 19 unsigned int inprogress, prev_inprogress, limit; 20 unsigned int ovlimit, completed, num_queued; 21 bool all_prev_completed; 22 23 num_queued = ACCESS_ONCE(dql->num_queued); 24 25 /* Can't complete more than what's in queue */ 26 BUG_ON(count > num_queued - dql->num_completed); 27 28 completed = dql->num_completed + count; 29 limit = dql->limit; 30 ovlimit = POSDIFF(num_queued - dql->num_completed, limit); 31 inprogress = num_queued - completed; 32 prev_inprogress = dql->prev_num_queued - dql->num_completed; 33 all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued); 34 35 if ((ovlimit && !inprogress) || 36 (dql->prev_ovlimit && all_prev_completed)) { 37 /* 38 * Queue considered starved if: 39 * - The queue was over-limit in the last interval, 40 * and there is no more data in the queue. 41 * OR 42 * - The queue was over-limit in the previous interval and 43 * when enqueuing it was possible that all queued data 44 * had been consumed. This covers the case when queue 45 * may have becomes starved between completion processing 46 * running and next time enqueue was scheduled. 47 * 48 * When queue is starved increase the limit by the amount 49 * of bytes both sent and completed in the last interval, 50 * plus any previous over-limit. 51 */ 52 limit += POSDIFF(completed, dql->prev_num_queued) + 53 dql->prev_ovlimit; 54 dql->slack_start_time = jiffies; 55 dql->lowest_slack = UINT_MAX; 56 } else if (inprogress && prev_inprogress && !all_prev_completed) { 57 /* 58 * Queue was not starved, check if the limit can be decreased. 59 * A decrease is only considered if the queue has been busy in 60 * the whole interval (the check above). 61 * 62 * If there is slack, the amount of execess data queued above 63 * the the amount needed to prevent starvation, the queue limit 64 * can be decreased. To avoid hysteresis we consider the 65 * minimum amount of slack found over several iterations of the 66 * completion routine. 67 */ 68 unsigned int slack, slack_last_objs; 69 70 /* 71 * Slack is the maximum of 72 * - The queue limit plus previous over-limit minus twice 73 * the number of objects completed. Note that two times 74 * number of completed bytes is a basis for an upper bound 75 * of the limit. 76 * - Portion of objects in the last queuing operation that 77 * was not part of non-zero previous over-limit. That is 78 * "round down" by non-overlimit portion of the last 79 * queueing operation. 80 */ 81 slack = POSDIFF(limit + dql->prev_ovlimit, 82 2 * (completed - dql->num_completed)); 83 slack_last_objs = dql->prev_ovlimit ? 84 POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0; 85 86 slack = max(slack, slack_last_objs); 87 88 if (slack < dql->lowest_slack) 89 dql->lowest_slack = slack; 90 91 if (time_after(jiffies, 92 dql->slack_start_time + dql->slack_hold_time)) { 93 limit = POSDIFF(limit, dql->lowest_slack); 94 dql->slack_start_time = jiffies; 95 dql->lowest_slack = UINT_MAX; 96 } 97 } 98 99 /* Enforce bounds on limit */ 100 limit = clamp(limit, dql->min_limit, dql->max_limit); 101 102 if (limit != dql->limit) { 103 dql->limit = limit; 104 ovlimit = 0; 105 } 106 107 dql->adj_limit = limit + completed; 108 dql->prev_ovlimit = ovlimit; 109 dql->prev_last_obj_cnt = dql->last_obj_cnt; 110 dql->num_completed = completed; 111 dql->prev_num_queued = num_queued; 112 } 113 EXPORT_SYMBOL(dql_completed); 114 115 void dql_reset(struct dql *dql) 116 { 117 /* Reset all dynamic values */ 118 dql->limit = 0; 119 dql->num_queued = 0; 120 dql->num_completed = 0; 121 dql->last_obj_cnt = 0; 122 dql->prev_num_queued = 0; 123 dql->prev_last_obj_cnt = 0; 124 dql->prev_ovlimit = 0; 125 dql->lowest_slack = UINT_MAX; 126 dql->slack_start_time = jiffies; 127 } 128 EXPORT_SYMBOL(dql_reset); 129 130 int dql_init(struct dql *dql, unsigned hold_time) 131 { 132 dql->max_limit = DQL_MAX_LIMIT; 133 dql->min_limit = 0; 134 dql->slack_hold_time = hold_time; 135 dql_reset(dql); 136 return 0; 137 } 138 EXPORT_SYMBOL(dql_init); 139