block: leave the request timeout timer running even on an empty list

For sync IO, we'll often do them serialized. This means we'll be touching
the queue timer for every IO, as opposed to only occasionally like we
do for queued IO. Instead of deleting the timer when the last request
is removed, just let continue running. If a new request comes up soon
we then don't have to readd the timer again. If no new requests arrive,
the timer will expire without side effect later.

This improves high iops sync IO by ~1%.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/blk-core.c

@@ -404,6 +404,7 @@ EXPORT_SYMBOL(blk_stop_queue);
 void blk_sync_queue(struct request_queue *q)
 {
 	del_timer_sync(&q->unplug_timer);
+	del_timer_sync(&q->timeout);
 	kblockd_flush_work(&q->unplug_work);
 }
 EXPORT_SYMBOL(blk_sync_queue);

block/blk-timeout.c

@@ -73,11 +73,7 @@ ssize_t part_timeout_store(struct device *dev, struct device_attribute *attr,
  */
 void blk_delete_timer(struct request *req)
 {
-	struct request_queue *q = req->q;
-
 	list_del_init(&req->timeout_list);
-	if (list_empty(&q->timeout_list))
-		del_timer(&q->timeout);
 }
 
 static void blk_rq_timed_out(struct request *req)