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