Re: [PATCH 2/4] writeback: quit background/periodic work when other works are enqueued

[Wu Fengguang <fengguang.wu@intel.com>]

Hi Jan,

Sorry for the long delay!

On Tue, Sep 14, 2010 at 08:40:33PM +0800, Jan Kara wrote:
>   Hi,
> 
> On Mon 13-09-10 20:31:12, Wu Fengguang wrote:
> >  From: Jan Kara <jack@suse.cz>
> > 
> > Background writeback and kupdate-style writeback are easily livelockable
> > (from a definition of their target). This is inconvenient because it can
> > make sync(1) stall forever waiting on its queued work to be finished.
> > Fix the problem by interrupting background and kupdate writeback if there
> > is some other work to do. We can return to them after completing all the
> > queued work.
>   I actually have a slightly updated version with a better changelog:
> 
> Background writeback are easily livelockable (from a definition of their
> target). This is inconvenient because it can make sync(1) stall forever waiting
> on its queued work to be finished. 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.
> 
>   Could you please update it? Thanks.
> 								Honza
> 
> PS: I've also attached the full patch if that's more convenient for you.

You patches are more complete than mine, so let's use them. However I
do prefer to have a standalone wb_check_background_flush() that is
called _after_ wb_check_old_data_flush(). This helps make the writeout
a bit more ordered and the separation itself looks a bit more clean to
me.

Followed are the slightly updated patches. IMHO they are straightforward
fixes that could be merged before other writeback changes.

Thanks,
Fengguang

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