[PATCH 1/2] loop: Prevent that an I/O scheduler is assigned

[Bart Van Assche <bvanassche@acm.org>]

Loop devices have a single hardware queue. Hence, the block layer function
elevator_get_default() selects the mq-deadline scheduler for loop devices.
Using the mq-deadline scheduler or any other I/O scheduler for loop devices
incurs unnecessary overhead. Make the loop driver pass the flag
BLK_MQ_F_NOSCHED to the block layer core such that no I/O scheduler can be
associated with block devices. This approach has an advantage compared to
letting udevd change the loop I/O scheduler to none, namely that
synchronize_rcu() does not get called.

It is intentional that the flag BLK_MQ_F_SHOULD_MERGE is preserved.

This patch reduces the Android boot time on my test setup with 0.5 seconds.

Cc: Ming Lei <ming.lei@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Martijn Coenen <maco@android.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/block/loop.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index f8486d9b75a4..9fca3ab3988d 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -2333,7 +2333,8 @@ static int loop_add(int i)
 	lo->tag_set.queue_depth = 128;
 	lo->tag_set.numa_node = NUMA_NO_NODE;
 	lo->tag_set.cmd_size = sizeof(struct loop_cmd);
-	lo->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_STACKING;
+	lo->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_STACKING |
+		BLK_MQ_F_NO_SCHED;
 	lo->tag_set.driver_data = lo;
 
 	err = blk_mq_alloc_tag_set(&lo->tag_set);