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SubjectRe: [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@intel.com> wrote:
>

I find the description to be somewhat incomplete...

> From: Jan Kara <jack@suse.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@suse.cz>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.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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