linux-kernel - Re: [PATCH] memory-hotplug: fix zone stat mismatch

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Message-ID: <20120920023053.GD13234@bbox>
Date:	Thu, 20 Sep 2012 11:30:53 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Wen Congyang <wency@...fujitsu.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	Shaohua Li <shli@...ionio.com>
Subject: Re: [PATCH] memory-hotplug: fix zone stat mismatch

On Thu, Sep 20, 2012 at 09:17:43AM +0800, Wen Congyang wrote:
> At 09/20/2012 04:17 AM, Minchan Kim Wrote:
> > Hi KOSAKI,
> > 
> > On Wed, Sep 19, 2012 at 02:05:20PM -0400, KOSAKI Motohiro wrote:
> >> On Wed, Sep 19, 2012 at 3:29 AM, Minchan Kim <minchan@...nel.org> wrote:
> >>> During memory-hotplug stress test, I found NR_ISOLATED_[ANON|FILE]
> >>> are increasing so that kernel are hang out.
> >>>
> >>> The cause is that when we do memory-hotadd after memory-remove,
> >>> __zone_pcp_update clear out zone's ZONE_STAT_ITEMS in setup_pageset
> >>> without draining vm_stat_diff of all CPU.
> >>>
> >>> This patch fixes it.
> >>
> >> zone_pcp_update() is called from online pages path. but IMHO,
> >> the statistics should be drained offline path. isn't it?
> > 
> > It isn't necessary because statistics is right until we reset it to zero
> > in online path.
> > Do you have something on your mind that we have to drain it in offline path?
> 
> When a node is offlined and onlined again. We create node_data[i] in the

I would like to clarify your word.
Create or recreate?
Why I have a question is as I look over the source code, hotadd_new_pgdat
seem to be called if we do hotadd *new* memory. It's not the case for
offline and online again you mentioned. If you're right, I should find 
arch_free_nodedata to free pgdat when node is disappear but I can't find it.
Do I miss something?


> function hotadd_new_pgdat(), and we will lost the statistics stored in
> zone->pageset. So we should drain it in offline path.

Even we drain in offline patch, it still has a problem.

1. offline
2. drain -> OKAY 
3. schedule
4. Process A increase zone stat
5. Process B increase zone stat
6. online
7. reset it -> we ends up lost zone stat counter which is modified between 2-6


> 
> Thanks
> Wen Congyang
> 
> > 
> >>
> >> thanks.
> > 
> 
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-- 
Kind regards,
Minchan Kim
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