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

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Message-ID: <505A89A8.8070008@cn.fujitsu.com>
Date:	Thu, 20 Sep 2012 11:12:40 +0800
From:	Wen Congyang <wency@...fujitsu.com>
To:	Minchan Kim <minchan@...nel.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...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>,
	Shaohua Li <shli@...ionio.com>
Subject: Re: [PATCH] memory-hotplug: fix zone stat mismatch

At 09/20/2012 10:30 AM, Minchan Kim Wrote:
> 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?

Hmm, when a memory is removed, we don't do cleanup now. We(Fujitsu) posted
a patchset to do this:
https://lkml.org/lkml/2012/9/5/201

We don't free pgdat in this patchset now. We have two choice:
1. free pgdat
2. don't free it, and reuse it when it is onlined again

I'm not sure which choice is better.

> 
> 
>> 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
> 

I understand why you drain it in online path now. But it still should drain it
in offline path because if all pages in this zone are offlined, we will call
zone_pcp_reset() to reset zone's pcp. We should also drop it in the function
zone_pcp_reset().

Thanks
Wen Congyang

> 
>>
>> Thanks
>> Wen Congyang
>>
>>>
>>>>
>>>> thanks.
>>>
>>
>> --
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