linux-kernel - Re: regression in page writeback

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Date:	Wed, 23 Sep 2009 09:17:58 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"richard@....demon.co.uk" <richard@....demon.co.uk>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>
Subject: Re: regression in page writeback

On Wed, Sep 23, 2009 at 08:54:52AM +0800, Andrew Morton wrote:
> On Wed, 23 Sep 2009 08:22:20 +0800 Wu Fengguang <fengguang.wu@...el.com> wrote:
> 
> > Jens' per-bdi writeback has another improvement. In 2.6.31, when
> > superblocks A and B both have 100000 dirty pages, it will first
> > exhaust A's 100000 dirty pages before going on to sync B's.
> 
> That would only be true if someone broke 2.6.31.  Did they?
> 
> SYSCALL_DEFINE0(sync)
> {
> 	wakeup_pdflush(0);
> 	sync_filesystems(0);
> 	sync_filesystems(1);
> 	if (unlikely(laptop_mode))
> 		laptop_sync_completion();
> 	return 0;
> }
> 
> the sync_filesystems(0) is supposed to non-blockingly start IO against
> all devices.  It used to do that correctly.  But people mucked with it
> so perhaps it no longer does.

I'm referring to writeback_inodes(). Each invocation of which (to sync
4MB) will do the same iteration over superblocks A => B => C ... So if
A has dirty pages, it will always be served first.

So if wbc->bdi == NULL (which is true for kupdate/background sync), it
will have to first exhaust A before going on to B and C.

There are no "cursor" in the superblock level iterations.

sync wants to exhaust all new inodes in A,B,C anyway, and it has the
live lock prevention logic based on dirtied_when, so that's not a big
problem for sync.

Thanks,
Fengguang

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