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From: Ming Lei <tom.leiming@gmail.com>
Date: Tue, 4 Apr 2017 22:24:12 +0800
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Subject: Re: [PATCH] loop: Add PF_LESS_THROTTLE to block/loop device thread.
To: NeilBrown <neilb@suse.com>
Cc: Jens Axboe <axboe@fb.com>,
        linux-block <linux-block@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Apr 3, 2017 at 9:18 AM, NeilBrown <neilb@suse.com> wrote:
>
> 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.
>
> Signed-off-by: NeilBrown <neilb@suse.com>
> ---
>  drivers/block/loop.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 0ecb6461ed81..a7e1dd215fc2 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1694,8 +1694,11 @@ static void loop_queue_work(struct kthread_work *work)
>  {
>         struct loop_cmd *cmd =
>                 container_of(work, struct loop_cmd, work);
> +       int oldflags = current->flags & PF_LESS_THROTTLE;
>
> +       current->flags |= PF_LESS_THROTTLE;
>         loop_handle_cmd(cmd);
> +       current->flags = (current->flags & ~PF_LESS_THROTTLE) | oldflags;
>  }

You can do it against 'lo->worker_task' instead of doing it in each
loop_queue_work(),
and this flag needn't to be restored because the kernel thread is loop
specialized.


thanks,
Ming Lei