[PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device thread.

[NeilBrown]

When a filesystem is mounted from a loop device, writes are
throttled by balance_dirty_pages() twice: once when writing
to the filesystem and once when the loop_handle_cmd() writes
to the backing file.  This double-throttling can trigger
positive feedback loops that create significant delays.  The
throttling at the lower level is seen by the upper level as
a slow device, so it throttles extra hard.

The PF_LESS_THROTTLE flag was created to handle exactly this
circumstance, though with an NFS filesystem mounted from a
local NFS server.  It reduces the throttling on the lower
layer so that it can proceed largely unthrottled.

To demonstrate this, create a filesystem on a loop device
and write (e.g. with dd) several large files which combine
to consume significantly more than the limit set by
/proc/sys/vm/dirty_ratio or dirty_bytes.  Measure the total
time taken.

When I do this directly on a device (no loop device) the
total time for several runs (mkfs, mount, write 200 files,
umount) is fairly stable: 28-35 seconds.
When I do this over a loop device the times are much worse
and less stable.  52-460 seconds.  Half below 100seconds,
half above.
When I apply this patch, the times become stable again,
though not as fast as the no-loop-back case: 53-72 seconds.

There may be room for further improvement as the total overhead still
seems too high, but this is a big improvement.

Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---

I moved where the flag is set, thanks to suggestion from
Ming Lei.
I've preserved the *-by: tags I was offered despite the code
being different, as the concept is identical.

Thanks,
NeilBrown


 drivers/block/loop.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 0ecb6461ed81..44b3506fd086 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -852,6 +852,7 @@ static int loop_prepare_queue(struct loop_device *lo)
 	if (IS_ERR(lo->worker_task))
 		return -ENOMEM;
 	set_user_nice(lo->worker_task, MIN_NICE);
+	lo->worker_task->flags |= PF_LESS_THROTTLE;
 	return 0;
 }
 
-- 
2.12.2

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