From: Mike Snitzer <snitzer@redhat.com>
Date: Tue, 24 Feb 2015 16:03:22 +0000 (-0500)
Subject: dm: only run the queue on completion if congested or no requests pending
X-Git-Tag: KARO-TXA5-2015-06-26~78^2~20
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=df1b2e3655b435b25933c9a23bf3b27fe62d6030;p=karo-tx-linux.git

dm: only run the queue on completion if congested or no requests pending

On really fast storage it can be beneficial to delay running the
request_queue to allow the elevator more opportunity to merge requests.

Otherwise, it has been observed that requests are being sent to
q->request_fn much quicker than is ideal on IOPS-bound backends.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 43e0d1a85a60..7924c00e0716 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1024,10 +1024,13 @@ static void end_clone_bio(struct bio *clone, int error)
  */
 static void rq_completed(struct mapped_device *md, int rw, bool run_queue)
 {
+	int nr_requests_pending;
+
 	atomic_dec(&md->pending[rw]);
 
 	/* nudge anyone waiting on suspend queue */
-	if (!md_in_flight(md))
+	nr_requests_pending = md_in_flight(md);
+	if (!nr_requests_pending)
 		wake_up(&md->wait);
 
 	/*
@@ -1036,8 +1039,11 @@ static void rq_completed(struct mapped_device *md, int rw, bool run_queue)
 	 * back into ->request_fn() could deadlock attempting to grab the
 	 * queue lock again.
 	 */
-	if (run_queue)
-		blk_run_queue_async(md->queue);
+	if (run_queue) {
+		if (!nr_requests_pending ||
+		    (nr_requests_pending >= md->queue->nr_congestion_on))
+			blk_run_queue_async(md->queue);
+	}
 
 	/*
 	 * dm_put() must be at the end of this function. See the comment above