cfq-iosched: give busy sync queue no dispatch limit

If there are a sync and an async queue and the sync queue's think time
is small, we can ignore the sync queue's dispatch quantum. Because the
sync queue will always preempt the async queue, we don't need to care
about async's latency.  This can fix a performance regression of
aiostress test, which is introduced by commit f8ae6e3eb825. The issue
should exist even without the commit, but the commit amplifies the
impact.

The initial post does the same optimization for RT queue too, but since
I have no real workload for it, Vivek suggests to drop it.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Reviewed-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/cfq-iosched.c

@@ -237,6 +237,7 @@ struct cfq_data {
 	struct rb_root prio_trees[CFQ_PRIO_LISTS];
 
 	unsigned int busy_queues;
+	unsigned int busy_sync_queues;
 
 	int rq_in_driver;
 	int rq_in_flight[2];
@@ -1344,6 +1345,8 @@ static void cfq_add_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 	BUG_ON(cfq_cfqq_on_rr(cfqq));
 	cfq_mark_cfqq_on_rr(cfqq);
 	cfqd->busy_queues++;
+	if (cfq_cfqq_sync(cfqq))
+		cfqd->busy_sync_queues++;
 
 	cfq_resort_rr_list(cfqd, cfqq);
 }
@@ -1370,6 +1373,8 @@ static void cfq_del_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 	cfq_group_service_tree_del(cfqd, cfqq->cfqg);
 	BUG_ON(!cfqd->busy_queues);
 	cfqd->busy_queues--;
+	if (cfq_cfqq_sync(cfqq))
+		cfqd->busy_sync_queues--;
 }
 
 /*
@@ -2377,22 +2382,39 @@ static bool cfq_may_dispatch(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 	 * Does this cfqq already have too much IO in flight?
 	 */
 	if (cfqq->dispatched >= max_dispatch) {
+		bool promote_sync = false;
 		/*
 		 * idle queue must always only have a single IO in flight
 		 */
 		if (cfq_class_idle(cfqq))
 			return false;
 
+		/*
+		 * If there is only one sync queue, and its think time is
+		 * small, we can ignore async queue here and give the sync
+		 * queue no dispatch limit. The reason is a sync queue can
+		 * preempt async queue, limiting the sync queue doesn't make
+		 * sense. This is useful for aiostress test.
+		 */
+		if (cfq_cfqq_sync(cfqq) && cfqd->busy_sync_queues == 1) {
+			struct cfq_io_context *cic = RQ_CIC(cfqq->next_rq);
+
+			if (sample_valid(cic->ttime_samples) &&
+				cic->ttime_mean < cfqd->cfq_slice_idle)
+				promote_sync = true;
+		}
+
 		/*
 		 * We have other queues, don't allow more IO from this one
 		 */
-		if (cfqd->busy_queues > 1 && cfq_slice_used_soon(cfqd, cfqq))
+		if (cfqd->busy_queues > 1 && cfq_slice_used_soon(cfqd, cfqq) &&
+				!promote_sync)
 			return false;
 
 		/*
 		 * Sole queue user, no limit
 		 */
-		if (cfqd->busy_queues == 1)
+		if (cfqd->busy_queues == 1 || promote_sync)
 			max_dispatch = -1;
 		else
 			/*