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

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Date:	Fri, 23 Jul 2010 19:39:54 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
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-mm@...ck.org
Subject: Re: [PATCH 5/6] writeback: try more writeback as long as something
 was written

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
queue all inodes that are eligible for writeback...

								Honza


> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> ---
>  fs/fs-writeback.c |   19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
> 
> --- linux-next.orig/fs/fs-writeback.c	2010-07-22 13:07:51.000000000 +0800
> +++ linux-next/fs/fs-writeback.c	2010-07-22 13:07:54.000000000 +0800
> @@ -640,20 +640,23 @@ static long wb_writeback(struct bdi_writ
>  		wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write;
>  
>  		/*
> -		 * If we consumed everything, see if we have more
> +		 * Did we write something? Try for more
> +		 *
> +		 * This is needed _before_ the b_more_io test because the
> +		 * background writeback moves inodes to b_io and works on
> +		 * them in batches (in order to sync old pages first).  The
> +		 * completion of the current batch does not necessarily mean
> +		 * the overall work is done.
>  		 */
> -		if (wbc.nr_to_write <= 0)
> +		if (wbc.nr_to_write < MAX_WRITEBACK_PAGES)
>  			continue;
> +
>  		/*
> -		 * Didn't write everything and we don't have more IO, bail
> +		 * Nothing written and no more inodes for IO, bail
>  		 */
>  		if (list_empty(&wb->b_more_io))
>  			break;
> -		/*
> -		 * Did we write something? Try for more
> -		 */
> -		if (wbc.nr_to_write < MAX_WRITEBACK_PAGES)
> -			continue;
> +
>  		/*
>  		 * Nothing written. Wait for some inode to
>  		 * become available for writeback. Otherwise
> 
> 
> --
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-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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