iocost: improve nr_lagging handling

Some IOs may span multiple periods.  As latencies are collected on
completion, the inbetween periods won't register them and may
incorrectly decide to increase vrate.  nr_lagging tracks these IOs to
avoid those situations.  Currently, whenever there are IOs which are
spanning from the previous period, busy_level is reset to 0 if
negative thus suppressing vrate increase.

This has the following two problems.

* When latency target percentiles aren't set, vrate adjustment should
  only be governed by queue depth depletion; however, the current code
  keeps nr_lagging active which pulls in latency results and can keep
  down vrate unexpectedly.

* When lagging condition is detected, it resets the entire negative
  busy_level.  This turned out to be way too aggressive on some
  devices which sometimes experience extended latencies on a small
  subset of commands.  In addition, a lagging IO will be accounted as
  latency target miss on completion anyway and resetting busy_level
  amplifies its impact unnecessarily.

This patch fixes the above two problems by disabling nr_lagging
counting when latency target percentiles aren't set and blocking vrate
increases when there are lagging IOs while leaving busy_level as-is.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iocost.c

@@ -1407,7 +1407,8 @@ static void ioc_timer_fn(struct timer_list *timer)
 		 * comparing vdone against period start.  If lagging behind
 		 * IOs from past periods, don't increase vrate.
 		 */
-		if (!atomic_read(&iocg_to_blkg(iocg)->use_delay) &&
+		if ((ppm_rthr != MILLION || ppm_wthr != MILLION) &&
+		    !atomic_read(&iocg_to_blkg(iocg)->use_delay) &&
 		    time_after64(vtime, vdone) &&
 		    time_after64(vtime, now.vnow -
 				 MAX_LAGGING_PERIODS * period_vtime) &&
@@ -1537,21 +1538,23 @@ static void ioc_timer_fn(struct timer_list *timer)
 	    missed_ppm[WRITE] > ppm_wthr) {
 		ioc->busy_level = max(ioc->busy_level, 0);
 		ioc->busy_level++;
-	} else if (nr_lagging) {
-		ioc->busy_level = max(ioc->busy_level, 0);
-	} else if (nr_shortages && !nr_surpluses &&
-		   rq_wait_pct <= RQ_WAIT_BUSY_PCT * UNBUSY_THR_PCT / 100 &&
+	} else if (rq_wait_pct <= RQ_WAIT_BUSY_PCT * UNBUSY_THR_PCT / 100 &&
 		   missed_ppm[READ] <= ppm_rthr * UNBUSY_THR_PCT / 100 &&
 		   missed_ppm[WRITE] <= ppm_wthr * UNBUSY_THR_PCT / 100) {
-		ioc->busy_level = min(ioc->busy_level, 0);
-		ioc->busy_level--;
+		/* take action iff there is contention */
+		if (nr_shortages && !nr_lagging) {
+			ioc->busy_level = min(ioc->busy_level, 0);
+			/* redistribute surpluses first */
+			if (!nr_surpluses)
+				ioc->busy_level--;
+		}
 	} else {
 		ioc->busy_level = 0;
 	}
 
 	ioc->busy_level = clamp(ioc->busy_level, -1000, 1000);
 
-	if (ioc->busy_level) {
+	if (ioc->busy_level > 0 || (ioc->busy_level < 0 && !nr_lagging)) {
 		u64 vrate = atomic64_read(&ioc->vtime_rate);
 		u64 vrate_min = ioc->vrate_min, vrate_max = ioc->vrate_max;