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