linux-kernel - Re: [PATCH 12/12] vmscan: Do not writeback pages in direct reclaim

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Message-Id: <20100616093958.00673123.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 16 Jun 2010 09:39:58 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Mel Gorman <mel@....ul.ie>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 12/12] vmscan: Do not writeback pages in direct reclaim

On Tue, 15 Jun 2010 20:29:49 -0400
Rik van Riel <riel@...hat.com> wrote:

> On 06/15/2010 08:17 PM, KAMEZAWA Hiroyuki wrote:
> > On Tue, 15 Jun 2010 15:16:01 +0100
> > Mel Gorman<mel@....ul.ie>  wrote:
> 
> >> But in turn, where is mem_cgroup_hierarchical_reclaim called from direct
> >> reclaim? It appears to be only called from the fault path or as a result
> >> of the memcg changing size.
> >>
> > yes. It's only called from
> > 	- page fault
> > 	- add_to_page_cache()
> >
> > I think we'll see no stack problem. Now, memcg doesn't wakeup kswapd for
> > reclaiming memory, it needs direct writeback.
> 
> Of course, a memcg page fault could still be triggered
> from copy_to_user or copy_from_user, with a fairly
> arbitrary stack frame above...
> 

Hmm. But I don't expect copy_from/to_user is called in very deep stack.

Should I prepare a thread for reclaiming memcg pages ?
Because we shouldn't limit kswapd's cpu time by CFS cgroup, waking up
kswapd just because "a memcg hit limits" isn't fun. 

Hmm, or do you recommend no-dirty-page-writeback when a memcg hits limit ?
Maybe we'll see much swaps.

I want to go with this for a while, changing memcg's behavior will took
some amounts of time, there are only a few developpers.

Thanks,
-Kame

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