[PATCH]cfq-iosched: don't stop async queue with async requests pending

[Shaohua Li <shaohua.li@intel.com>]

My SSD speed of direct write is about 80m/s, while I test page writeback,
the speed can only go to 68m/s. Below patch fixes this.
It appears we missused cfq_should_idle in cfq_may_dispatch. cfq_should_idle
means a queue should idle because it's seekless sync queue or it's the last queue,
which is to maintain service tree time slice. So it doesn't mean the
last queue is always a sync queue. If the last queue is asyn queue,
we definitely shouldn't stop dispatch requests because of pending async
requests.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 918c7fd..8198079 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -2222,7 +2222,8 @@ static bool cfq_may_dispatch(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 	/*
 	 * Drain async requests before we start sync IO
 	 */
-	if (cfq_should_idle(cfqd, cfqq) && cfqd->rq_in_driver[BLK_RW_ASYNC])
+	if (cfq_cfqq_sync(cfqq) && cfq_should_idle(cfqd, cfqq)
+		&& cfqd->rq_in_driver[BLK_RW_ASYNC])
 		return false;
 
 	/*