loop: Select I/O scheduler 'none' from inside add_disk()

We noticed that the user interface of Android devices becomes very slow
under memory pressure. This is because Android uses the zram driver on top
of the loop driver for swapping, because under memory pressure the swap
code alternates reads and writes quickly, because mq-deadline is the
default scheduler for loop devices and because mq-deadline delays writes by
five seconds for such a workload with default settings. Fix this by making
the kernel select I/O scheduler 'none' from inside add_disk() for loop
devices. This default can be overridden at any time from user space,
e.g. via a udev rule. This approach has an advantage compared to changing
the I/O scheduler from userspace from 'mq-deadline' into 'none', namely
that synchronize_rcu() does not get called.

This patch changes the default I/O scheduler for loop devices from
'mq-deadline' into 'none'.

Additionally, this patch reduces the Android boot time on my test setup
with 0.5 seconds compared to configuring the loop I/O scheduler from user
space.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Martijn Coenen <maco@android.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20210805174200.3250718-3-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index f8486d9b75a49d6ea7de0364689ba2b5e6494b25..fa1c298a8cfbb87c7d66bf4117de7f84c9b0b42c 100644 (file)
--- 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_BY_DEFAULT;
        lo->tag_set.driver_data = lo;
 
        err = blk_mq_alloc_tag_set(&lo->tag_set);