[PATCH] cfq-iosched: check busy queues before deciding we are idle

For just one busy queue (like async write out), we often overlooked
that we could queue more io and decided we were idle instead. This causes
us quite a bit of performance loss.
Signed-off-by: Jens Axboe <axboe@suse.de>

block/cfq-iosched.c

@@ -878,6 +878,13 @@ static struct cfq_queue *cfq_set_active_queue(struct cfq_data *cfqd)
 	if (!list_empty(&cfqd->cur_rr) || cfq_get_next_prio_level(cfqd) != -1)
 		cfqq = list_entry_cfqq(cfqd->cur_rr.next);
 
+	/*
+	 * If no new queues are available, check if the busy list has some
+	 * before falling back to idle io.
+	 */
+	if (!cfqq && !list_empty(&cfqd->busy_rr))
+		cfqq = list_entry_cfqq(cfqd->busy_rr.next);
+
 	/*
 	 * if we have idle queues and no rt or be queues had pending
 	 * requests, either allow immediate service if the grace period