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