FreeBSD/Linux Kernel Cross Reference
sys/lib/dynamic_queue_limits.c

     /*
      * Dynamic byte queue limits.  See include/linux/dynamic_queue_limits.h
      *
      * Copyright (c) 2011, Tom Herbert <therbert@google.com>
      */
     #include <linux/module.h>
     #include <linux/types.h>
     #include <linux/ctype.h>
     #include <linux/kernel.h>
    #include <linux/jiffies.h>
    #include <linux/dynamic_queue_limits.h>
    
    #define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
    #define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)
    
    /* Records completed count and recalculates the queue limit */
    void dql_completed(struct dql *dql, unsigned int count)
    {
            unsigned int inprogress, prev_inprogress, limit;
            unsigned int ovlimit, completed, num_queued;
            bool all_prev_completed;
    
            num_queued = ACCESS_ONCE(dql->num_queued);
    
            /* Can't complete more than what's in queue */
            BUG_ON(count > num_queued - dql->num_completed);
    
            completed = dql->num_completed + count;
            limit = dql->limit;
            ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
            inprogress = num_queued - completed;
            prev_inprogress = dql->prev_num_queued - dql->num_completed;
            all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
    
            if ((ovlimit && !inprogress) ||
                (dql->prev_ovlimit && all_prev_completed)) {
                    /*
                     * Queue considered starved if:
                     *   - The queue was over-limit in the last interval,
                     *     and there is no more data in the queue.
                     *  OR
                     *   - The queue was over-limit in the previous interval and
                     *     when enqueuing it was possible that all queued data
                     *     had been consumed.  This covers the case when queue
                     *     may have becomes starved between completion processing
                     *     running and next time enqueue was scheduled.
                     *
                     *     When queue is starved increase the limit by the amount
                     *     of bytes both sent and completed in the last interval,
                     *     plus any previous over-limit.
                     */
                    limit += POSDIFF(completed, dql->prev_num_queued) +
                         dql->prev_ovlimit;
                    dql->slack_start_time = jiffies;
                    dql->lowest_slack = UINT_MAX;
            } else if (inprogress && prev_inprogress && !all_prev_completed) {
                    /*
                     * Queue was not starved, check if the limit can be decreased.
                     * A decrease is only considered if the queue has been busy in
                     * the whole interval (the check above).
                     *
                     * If there is slack, the amount of execess data queued above
                     * the the amount needed to prevent starvation, the queue limit
                     * can be decreased.  To avoid hysteresis we consider the
                     * minimum amount of slack found over several iterations of the
                     * completion routine.
                     */
                    unsigned int slack, slack_last_objs;
    
                    /*
                     * Slack is the maximum of
                     *   - The queue limit plus previous over-limit minus twice
                     *     the number of objects completed.  Note that two times
                     *     number of completed bytes is a basis for an upper bound
                     *     of the limit.
                     *   - Portion of objects in the last queuing operation that
                     *     was not part of non-zero previous over-limit.  That is
                     *     "round down" by non-overlimit portion of the last
                     *     queueing operation.
                     */
                    slack = POSDIFF(limit + dql->prev_ovlimit,
                        2 * (completed - dql->num_completed));
                    slack_last_objs = dql->prev_ovlimit ?
                        POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
    
                    slack = max(slack, slack_last_objs);
    
                    if (slack < dql->lowest_slack)
                            dql->lowest_slack = slack;
    
                    if (time_after(jiffies,
                                   dql->slack_start_time + dql->slack_hold_time)) {
                            limit = POSDIFF(limit, dql->lowest_slack);
                            dql->slack_start_time = jiffies;
                            dql->lowest_slack = UINT_MAX;
                    }
            }
    
            /* Enforce bounds on limit */
           limit = clamp(limit, dql->min_limit, dql->max_limit);
   
           if (limit != dql->limit) {
                   dql->limit = limit;
                   ovlimit = 0;
           }
   
           dql->adj_limit = limit + completed;
           dql->prev_ovlimit = ovlimit;
           dql->prev_last_obj_cnt = dql->last_obj_cnt;
           dql->num_completed = completed;
           dql->prev_num_queued = num_queued;
   }
   EXPORT_SYMBOL(dql_completed);
   
   void dql_reset(struct dql *dql)
   {
           /* Reset all dynamic values */
           dql->limit = 0;
           dql->num_queued = 0;
           dql->num_completed = 0;
           dql->last_obj_cnt = 0;
           dql->prev_num_queued = 0;
           dql->prev_last_obj_cnt = 0;
           dql->prev_ovlimit = 0;
           dql->lowest_slack = UINT_MAX;
           dql->slack_start_time = jiffies;
   }
   EXPORT_SYMBOL(dql_reset);
   
   int dql_init(struct dql *dql, unsigned hold_time)
   {
           dql->max_limit = DQL_MAX_LIMIT;
           dql->min_limit = 0;
           dql->slack_hold_time = hold_time;
           dql_reset(dql);
           return 0;
   }
   EXPORT_SYMBOL(dql_init);

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