the-tree/lib/dynamic_queue_limits.c

0001 // SPDX-License-Identifier: GPL-2.0
0002 /*
0003  * Dynamic byte queue limits.  See include/linux/dynamic_queue_limits.h
0004  *
0005  * Copyright (c) 2011, Tom Herbert <therbert@google.com>
0006  */
0007 #include <linux/types.h>
0008 #include <linux/kernel.h>
0009 #include <linux/jiffies.h>
0010 #include <linux/dynamic_queue_limits.h>
0011 #include <linux/compiler.h>
0012 #include <linux/export.h>
0013
0014 #define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
0015 #define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)
0016
0017 /* Records completed count and recalculates the queue limit */
0018 void dql_completed(struct dql *dql, unsigned int count)
0019 {
0020     unsigned int inprogress, prev_inprogress, limit;
0021     unsigned int ovlimit, completed, num_queued;
0022     bool all_prev_completed;
0023
0024     num_queued = READ_ONCE(dql->num_queued);
0025
0026     /* Can't complete more than what's in queue */
0027     BUG_ON(count > num_queued - dql->num_completed);
0028
0029     completed = dql->num_completed + count;
0030     limit = dql->limit;
0031     ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
0032     inprogress = num_queued - completed;
0033     prev_inprogress = dql->prev_num_queued - dql->num_completed;
0034     all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
0035
0036     if ((ovlimit && !inprogress) ||
0037         (dql->prev_ovlimit && all_prev_completed)) {
0038         /*
0039          * Queue considered starved if:
0040          *   - The queue was over-limit in the last interval,
0041          *     and there is no more data in the queue.
0042          *  OR
0043          *   - The queue was over-limit in the previous interval and
0044          *     when enqueuing it was possible that all queued data
0045          *     had been consumed.  This covers the case when queue
0046          *     may have becomes starved between completion processing
0047          *     running and next time enqueue was scheduled.
0048          *
0049          *     When queue is starved increase the limit by the amount
0050          *     of bytes both sent and completed in the last interval,
0051          *     plus any previous over-limit.
0052          */
0053         limit += POSDIFF(completed, dql->prev_num_queued) +
0054              dql->prev_ovlimit;
0055         dql->slack_start_time = jiffies;
0056         dql->lowest_slack = UINT_MAX;
0057     } else if (inprogress && prev_inprogress && !all_prev_completed) {
0058         /*
0059          * Queue was not starved, check if the limit can be decreased.
0060          * A decrease is only considered if the queue has been busy in
0061          * the whole interval (the check above).
0062          *
0063          * If there is slack, the amount of excess data queued above
0064          * the amount needed to prevent starvation, the queue limit
0065          * can be decreased.  To avoid hysteresis we consider the
0066          * minimum amount of slack found over several iterations of the
0067          * completion routine.
0068          */
0069         unsigned int slack, slack_last_objs;
0070
0071         /*
0072          * Slack is the maximum of
0073          *   - The queue limit plus previous over-limit minus twice
0074          *     the number of objects completed.  Note that two times
0075          *     number of completed bytes is a basis for an upper bound
0076          *     of the limit.
0077          *   - Portion of objects in the last queuing operation that
0078          *     was not part of non-zero previous over-limit.  That is
0079          *     "round down" by non-overlimit portion of the last
0080          *     queueing operation.
0081          */
0082         slack = POSDIFF(limit + dql->prev_ovlimit,
0083             2 * (completed - dql->num_completed));
0084         slack_last_objs = dql->prev_ovlimit ?
0085             POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
0086
0087         slack = max(slack, slack_last_objs);
0088
0089         if (slack < dql->lowest_slack)
0090             dql->lowest_slack = slack;
0091
0092         if (time_after(jiffies,
0093                    dql->slack_start_time + dql->slack_hold_time)) {
0094             limit = POSDIFF(limit, dql->lowest_slack);
0095             dql->slack_start_time = jiffies;
0096             dql->lowest_slack = UINT_MAX;
0097         }
0098     }
0099
0100     /* Enforce bounds on limit */
0101     limit = clamp(limit, dql->min_limit, dql->max_limit);
0102
0103     if (limit != dql->limit) {
0104         dql->limit = limit;
0105         ovlimit = 0;
0106     }
0107
0108     dql->adj_limit = limit + completed;
0109     dql->prev_ovlimit = ovlimit;
0110     dql->prev_last_obj_cnt = dql->last_obj_cnt;
0111     dql->num_completed = completed;
0112     dql->prev_num_queued = num_queued;
0113 }
0114 EXPORT_SYMBOL(dql_completed);
0115
0116 void dql_reset(struct dql *dql)
0117 {
0118     /* Reset all dynamic values */
0119     dql->limit = 0;
0120     dql->num_queued = 0;
0121     dql->num_completed = 0;
0122     dql->last_obj_cnt = 0;
0123     dql->prev_num_queued = 0;
0124     dql->prev_last_obj_cnt = 0;
0125     dql->prev_ovlimit = 0;
0126     dql->lowest_slack = UINT_MAX;
0127     dql->slack_start_time = jiffies;
0128 }
0129 EXPORT_SYMBOL(dql_reset);
0130
0131 void dql_init(struct dql *dql, unsigned int hold_time)
0132 {
0133     dql->max_limit = DQL_MAX_LIMIT;
0134     dql->min_limit = 0;
0135     dql->slack_hold_time = hold_time;
0136     dql_reset(dql);
0137 }
0138 EXPORT_SYMBOL(dql_init);