linux-kernel - Re: [PATCH 2/3] mm, slub: count freed pages via rcu as this task's reclaimed

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Date:	Wed, 10 Apr 2013 23:24:38 +0900
From:	JoonSoo Kim <js1304@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan@...nel.org>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>
Subject: Re: [PATCH 2/3] mm, slub: count freed pages via rcu as this task's reclaimed_slab

2013/4/10 Christoph Lameter <cl@...ux.com>:
> On Wed, 10 Apr 2013, Joonsoo Kim wrote:
>
>> Hello, Christoph.
>>
>> On Tue, Apr 09, 2013 at 02:28:06PM +0000, Christoph Lameter wrote:
>> > On Tue, 9 Apr 2013, Joonsoo Kim wrote:
>> >
>> > > Currently, freed pages via rcu is not counted for reclaimed_slab, because
>> > > it is freed in rcu context, not current task context. But, this free is
>> > > initiated by this task, so counting this into this task's reclaimed_slab
>> > > is meaningful to decide whether we continue reclaim, or not.
>> > > So change code to count these pages for this task's reclaimed_slab.
>> >
>> > slab->reclaim_state guides the reclaim actions in vmscan.c. With this
>> > patch slab->reclaim_state could get quite a high value without new pages being
>> > available for allocation. slab->reclaim_state will only be updated
>> > when the RCU period ends.
>>
>> Okay.
>>
>> In addition, there is a little place who use SLAB_DESTROY_BY_RCU.
>> I will drop this patch[2/3] and [3/3] for next spin.
>
> What you have discoverd is an issue that we have so far overlooked. Could
> you add comments to both places explaining the situation?

Yes, I can.

> RCU is used for
> some inode and the dentry cache. Failing to account for these frees could
> pose a problem. One solution would be to ensure that we get through an RCU
> quiescent period in the slabs reclaim. If we can ensure that then your
> patch may be ok.

Hmm... I don't perfectly understand RCU code and it's quiescent
period. But, yes, it
can be one of possible solutions in my quick thought. Currently, I
have no ability to
do that, so I skip to think about this.

Thanks.

>
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