linux-kernel - Re: [PATCH 3/5] writeback: stop background/kupdate works from livelocking other works

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Message-Id: <20101109131310.f442d210.akpm@linux-foundation.org>
Date:	Tue, 9 Nov 2010 13:13:10 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@....de>,
	Jan Engelhardt <jengelh@...ozas.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] writeback: stop background/kupdate works from
 livelocking other works

On Tue, 09 Nov 2010 07:09:19 +0800
Wu Fengguang <fengguang.wu@...el.com> wrote:
>

I find the description to be somewhat incomplete...

> From: Jan Kara <jack@...e.cz>
> 
> Background writeback are easily livelockable (from a definition of their
> target).

*why* is background writeback easily livelockable?  Under which
circumstances does this happen and how does it come about?

> This is inconvenient because it can make sync(1) stall forever waiting
> on its queued work to be finished.

Again, why?  Because there are works queued from the flusher thread,
but that thread is stuck in a livelocked state in <unspecified code
location> so it is unable to service the other works?  But the pocess
which called sync() will as a last resort itself perform all the
required IO, will it not?  If so, how can it livelock?

> Generally, when a flusher thread has
> some work queued, someone submitted the work to achieve a goal more specific
> than what background writeback does. So it makes sense to give it a priority
> over a generic page cleaning.
> 
> Thus we interrupt background writeback if there is some other work to do. We
> return to the background writeback after completing all the queued work.
> 
> Signed-off-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> ---
>  fs/fs-writeback.c |    9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> --- linux-next.orig/fs/fs-writeback.c	2010-11-07 21:56:42.000000000 +0800
> +++ linux-next/fs/fs-writeback.c	2010-11-07 22:00:51.000000000 +0800
> @@ -651,6 +651,15 @@ static long wb_writeback(struct bdi_writ
>  			break;
>  
>  		/*
> +		 * Background writeout and kupdate-style writeback are
> +		 * easily livelockable. Stop them if there is other work
> +		 * to do so that e.g. sync can proceed.
> +		 */
> +		if ((work->for_background || work->for_kupdate) &&
> +		    !list_empty(&wb->bdi->work_list))
> +			break;
> +
> +		/*
>  		 * For background writeout, stop when we are below the
>  		 * background dirty threshold
>  		 */

So...  what prevents higher priority works (eg, sync(1)) from
livelocking or seriously retarding background or kudate writeout?

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