linux-kernel - Re: [PATCH 3/3] writeback: avoid extra sync work at enqueue time

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Message-ID: <20110504212427.GI6968@quack.suse.cz>
Date:	Wed, 4 May 2011 23:24:27 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@....de>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 3/3] writeback: avoid extra sync work at enqueue time

On Mon 02-05-11 11:17:53, Wu Fengguang wrote:
> This removes writeback_control.wb_start and does more straightforward
> sync livelock prevention by setting .older_than_this to prevent extra
> inodes from being enqueued in the first place.
> 
> --- linux-next.orig/fs/fs-writeback.c	2011-05-02 11:17:24.000000000 +0800
> +++ linux-next/fs/fs-writeback.c	2011-05-02 11:17:27.000000000 +0800
> @@ -683,10 +672,12 @@ static long wb_writeback(struct bdi_writ
>  	 *                   (quickly) tag currently dirty pages
>  	 *                   (maybe slowly) sync all tagged pages
>  	 */
> -	if (wbc.sync_mode == WB_SYNC_ALL || wbc.tagged_sync)
> +	if (wbc.sync_mode == WB_SYNC_ALL || wbc.tagged_sync) {
>  		write_chunk = LONG_MAX;
> +		oldest_jif = jiffies;
> +		wbc.older_than_this = &oldest_jif;
> +	}
  What are the implications of not doing dirty-time livelock avoidance for
other types of writeback? Is that a mistake? I'd prefer to have in
wb_writeback():
if (wbc.for_kupdate)
	oldest_jif = jiffies - msecs_to_jiffies(dirty_expire_interval * 10);
else
	oldest_jif = jiffies;
wbc.older_than_this = &oldest_jif;

And when you have this, you can make wbc.older_than_this just a plain
number and remove all those checks for wbc.older_than_this == NULL.

									Honza

-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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