blk-iolatency: keep track of previous windows stats

We apply a smoothing to the scale changes in order to keep sawtoothy
behavior from occurring. However our window for checking if we've
missed our target can sometimes be lower than the smoothing interval
(500ms), especially on faster drives like ssd's. In order to deal with
this keep track of the running tally of the previous intervals that we
threw away because we had already done a scale event recently.

This is needed for the ssd case as these low latency drives will have
bursts of latency, and if it happens to be ok for the window that
directly follows the opening of the scale window we could unthrottle
when previous windows we were missing our target.

Signed-off-by: Josef Bacik <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/blk-iolatency.c

···
 struct iolatency_grp {
 	struct blkg_policy_data pd;
 	struct latency_stat __percpu *stats;
+	struct latency_stat cur_stat;
 	struct blk_iolatency *blkiolat;
 	struct rq_depth rq_depth;
 	struct rq_wait rq_wait;
···
 
 	/* Somebody beat us to the punch, just bail. */
 	spin_lock_irqsave(&lat_info->lock, flags);
+
+	latency_stat_sum(iolat, &iolat->cur_stat, &stat);
 	lat_info->nr_samples -= iolat->nr_samples;
-	lat_info->nr_samples += latency_stat_samples(iolat, &stat);
-	iolat->nr_samples = latency_stat_samples(iolat, &stat);
+	lat_info->nr_samples += latency_stat_samples(iolat, &iolat->cur_stat);
+	iolat->nr_samples = latency_stat_samples(iolat, &iolat->cur_stat);
 
 	if ((lat_info->last_scale_event >= now ||
-	    now - lat_info->last_scale_event < BLKIOLATENCY_MIN_ADJUST_TIME) &&
-	    lat_info->scale_lat <= iolat->min_lat_nsec)
+	    now - lat_info->last_scale_event < BLKIOLATENCY_MIN_ADJUST_TIME))
 		goto out;
 
-	if (latency_sum_ok(iolat, &stat)) {
-		if (latency_stat_samples(iolat, &stat) <
+	if (latency_sum_ok(iolat, &iolat->cur_stat) &&
+	    latency_sum_ok(iolat, &stat)) {
+		if (latency_stat_samples(iolat, &iolat->cur_stat) <
 		    BLKIOLATENCY_MIN_GOOD_SAMPLES)
 			goto out;
 		if (lat_info->scale_grp == iolat) {
 			lat_info->last_scale_event = now;
 			scale_cookie_change(iolat->blkiolat, lat_info, true);
 		}
-	} else {
+	} else if (lat_info->scale_lat == 0 ||
+		   lat_info->scale_lat >= iolat->min_lat_nsec) {
 		lat_info->last_scale_event = now;
 		if (!lat_info->scale_grp ||
 		    lat_info->scale_lat > iolat->min_lat_nsec) {
···
 		}
 		scale_cookie_change(iolat->blkiolat, lat_info, false);
 	}
+	latency_stat_init(iolat, &iolat->cur_stat);
 out:
 	spin_unlock_irqrestore(&lat_info->lock, flags);
 }
···
 		latency_stat_init(iolat, stat);
 	}
 
+	latency_stat_init(iolat, &iolat->cur_stat);
 	rq_wait_init(&iolat->rq_wait);
 	spin_lock_init(&iolat->child_lat.lock);
 	iolat->rq_depth.queue_depth = blkg->q->nr_requests;