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

On 8/2/21 6:54 PM, Ming Lei wrote:

On Mon, Aug 02, 2021 at 05:01:59PM -0700, Bart Van Assche wrote:

quoted

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?

Hi Ming,

The software process called apexd creates multiple loop devices while
the device is booting. Using BLK_MQ_F_NO_SCHED is faster than letting
apexd change the I/O scheduler from mq-deadline into 'none' since the
latter involves calling synchronize_rcu() once per loop device.

quoted

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")

Setting BLK_MQ_F_NO_SCHED only for buffered I/O mode could be tricky
since the loop driver creates a request queue before the I/O mode is
configured. Anyway, I will look into this.

Bart.