linux-kernel - Re: [PATCH 5/6] writeback: try more writeback as long as something was written

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Message-ID: <20100726123937.GB11947@localhost>
Date:	Mon, 26 Jul 2010 20:39:37 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	Mel Gorman <mel@....ul.ie>,
	Chris Mason <chris.mason@...cle.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 5/6] writeback: try more writeback as long as something
 was written

On Sat, Jul 24, 2010 at 01:39:54AM +0800, Jan Kara wrote:
> On Thu 22-07-10 13:09:33, Wu Fengguang wrote:
> > writeback_inodes_wb()/__writeback_inodes_sb() are not agressive in that
> > they only populate b_io when necessary at entrance time. When the queued
> > set of inodes are all synced, they just return, possibly with
> > wbc.nr_to_write > 0.
> > 
> > For kupdate and background writeback, there may be more eligible inodes
> > sitting in b_dirty when the current set of b_io inodes are completed. So
> > it is necessary to try another round of writeback as long as we made some
> > progress in this round. When there are no more eligible inodes, no more
> > inodes will be enqueued in queue_io(), hence nothing could/will be
> > synced and we may safely bail.
> > 
> > This will livelock sync when there are heavy dirtiers. However in that case
> > sync will already be livelocked w/o this patch, as the current livelock
> > avoidance code is virtually a no-op (for one thing, wb_time should be
> > set statically at sync start time and be used in move_expired_inodes()).
> > The sync livelock problem will be addressed in other patches.
>   Hmm, any reason why you don't solve this problem by just removing the
> condition before queue_io()? It would also make the logic simpler - always

Yeah I'll remove queue_io() in the coming sync livelock patchset.
This patchset does the below. Though awkward, it avoids unnecessary
behavior changes for non-background cases.

-       if (!wbc->for_kupdate || list_empty(&wb->b_io))
+       if (!(wbc->for_kupdate || wbc->for_background) || list_empty(&wb->b_io))
                queue_io(wb, wbc);


> queue all inodes that are eligible for writeback...

Thanks,
Fengguang
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