cfq-iosched: delay async IO dispatch, if sync IO was just done

o Do not allow more than max_dispatch requests from an async queue, if some
  sync request has finished recently. This is in the hope that sync activity
  is still going on in the system and we might receive a sync request soon.
  Most likely from a sync queue which finished a request and we did not enable
  idling on it.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/cfq-iosched.c

@@ -181,6 +181,8 @@ struct cfq_data {
 	 * Fallback dummy cfqq for extreme OOM conditions
 	 */
 	struct cfq_queue oom_cfqq;
+
+	unsigned long last_end_sync_rq;
 };
 
 enum cfqq_state_flags {
@@ -1314,6 +1316,8 @@ static int cfq_dispatch_requests(struct request_queue *q, int force)
 	 * Does this cfqq already have too much IO in flight?
 	 */
 	if (cfqq->dispatched >= max_dispatch) {
+		unsigned long load_at = cfqd->last_end_sync_rq + cfq_slice_sync;
+
 		/*
 		 * idle queue must always only have a single IO in flight
 		 */
@@ -1326,6 +1330,14 @@ static int cfq_dispatch_requests(struct request_queue *q, int force)
 		if (cfqd->busy_queues > 1)
 			return 0;
 
+		/*
+		 * If a sync request has completed recently, don't overload
+		 * the dispatch queue yet with async requests.
+		 */
+		if (cfqd->cfq_desktop && !cfq_cfqq_sync(cfqq)
+		    && time_before(jiffies, load_at))
+			return 0;
+
 		/*
 		 * we are the only queue, allow up to 4 times of 'quantum'
 		 */
@@ -2158,8 +2170,10 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
 	if (cfq_cfqq_sync(cfqq))
 		cfqd->sync_flight--;
 
-	if (sync)
+	if (sync) {
 		RQ_CIC(rq)->last_end_request = now;
+		cfqd->last_end_sync_rq = now;
+	}
 
 	/*
 	 * If this is the active queue, check if it needs to be expired,
@@ -2483,7 +2497,7 @@ static void *cfq_init_queue(struct request_queue *q)
 	cfqd->cfq_slice_idle = cfq_slice_idle;
 	cfqd->cfq_desktop = 1;
 	cfqd->hw_tag = 1;
-
+	cfqd->last_end_sync_rq = jiffies;
 	return cfqd;
 }