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[PATCH v3] loop: Add PF_LESS_THROTTLE to block/loop device thread.

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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@lst.de>
Reviewed-by: Ming Lei <tom.leiming@gmail.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: NeilBrown <neilb@suse.com>
---

Hi Jens,
 I think this version meets with everyone's approval.

Thanks,
NeilBrown


 drivers/block/loop.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 0ecb6461ed81..035b8651b8bf 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -844,10 +844,16 @@ static void loop_unprepare_queue(struct loop_device *lo)
 	kthread_stop(lo->worker_task);
 }
 
+static int loop_kthread_worker_fn(void *worker_ptr)
+{
+	current->flags |= PF_LESS_THROTTLE;
+	return kthread_worker_fn(worker_ptr);
+}
+
 static int loop_prepare_queue(struct loop_device *lo)
 {
 	kthread_init_worker(&lo->worker);
-	lo->worker_task = kthread_run(kthread_worker_fn,
+	lo->worker_task = kthread_run(loop_kthread_worker_fn,
 			&lo->worker, "loop%d", lo->lo_number);
 	if (IS_ERR(lo->worker_task))
 		return -ENOMEM;
-- 
2.12.2

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[PATCH] loop: Add PF_LESS_THROTTLE to block/loop device thread. NeilBrown <neilb@suse.com> - 2017-04-03 03:20 +0200
  Re: [PATCH] loop: Add PF_LESS_THROTTLE to block/loop device thread. Christoph Hellwig <hch@infradead.org> - 2017-04-04 09:20 +0200
    Re: [PATCH] loop: Add PF_LESS_THROTTLE to block/loop device thread. NeilBrown <neilb@suse.com> - 2017-04-05 06:30 +0200
      Re: [PATCH] loop: Add PF_LESS_THROTTLE to block/loop device thread. Ming Lei <tom.leiming@gmail.com> - 2017-04-05 07:20 +0200
  Re: [PATCH] loop: Add PF_LESS_THROTTLE to block/loop device thread. Michal Hocko <mhocko@kernel.org> - 2017-04-04 13:30 +0200
  Re: [PATCH] loop: Add PF_LESS_THROTTLE to block/loop device thread. Ming Lei <tom.leiming@gmail.com> - 2017-04-04 16:30 +0200
    Re: [PATCH] loop: Add PF_LESS_THROTTLE to block/loop device thread. NeilBrown <neilb@suse.com> - 2017-04-05 06:40 +0200
  [PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device thread. NeilBrown <neilb@suse.com> - 2017-04-05 06:40 +0200
    Re: [PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device thread. Ming Lei <tom.leiming@gmail.com> - 2017-04-05 07:10 +0200
    Re: [PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device  thread. Michal Hocko <mhocko@kernel.org> - 2017-04-05 09:20 +0200
      Re: [PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device  thread. Michal Hocko <mhocko@kernel.org> - 2017-04-05 09:40 +0200
        Re: [PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device thread. NeilBrown <neilb@suse.com> - 2017-04-06 04:30 +0200
          Re: [PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device  thread. Michal Hocko <mhocko@kernel.org> - 2017-04-06 09:00 +0200
            [PATCH v3] loop: Add PF_LESS_THROTTLE to block/loop device thread. NeilBrown <neilb@suse.com> - 2017-04-07 01:50 +0200

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