On Mon, Aug 02, 2021 at 05:01:59PM -0700, Bart Van Assche wrote:
> 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.
Can you investigate why none reduces Android boot time? Or reproduce &
understand it by a fio simulation on your setting?
>
> Cc: Ming Lei <ming.lei@xxxxxxxxxx>
> Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Martijn Coenen <maco@xxxxxxxxxxx>
> Cc: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
> ---
> 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;
Loop directio needs io merge, so it isn't good to set NO_SCHED
unconditionally, see:
40326d8a33d5 ("block/loop: allow request merge for directio mode")
Thanks,
Ming
