linux-kernel - Re: [PATCH 3/3] mm, mem-hotplug: update pcp->stat

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Date:	Tue, 12 Apr 2011 19:42:07 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Yasunori Goto <y-goto@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>, Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH 3/3] mm, mem-hotplug: update pcp->stat_threshold when
 memory hotplug occur

On Tue, Apr 12, 2011 at 7:34 PM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:
>> > No good stat_threshold might makes performance hurt.
>>
>> Yes. That's I want it.
>> My intention is that if you write down log fully, it can help much
>> newbies to understand the patch in future and it would be very clear
>> Andrew to merge it.
>>
>> What I want is following as.
>> ==
>>
>> Currently, memory hotplug doesn't updates pcp->stat_threashold.
>> Then, It ends up making the wrong stat_threshold and percpu_driftmark.
>>
>> It could make confusing zoneinfo or overhead by frequent draining.
>> Even when memory is low and kswapd is awake, it can mismatch between
>> the number of real free pages and vmstat NR_FREE_PAGES so that it can
>> result in the livelock. Please look at aa4548403 for more.
>>
>> This patch solves the issue.
>> ==
>
> Now, wakeup_kswapd() are using zone_watermark_ok_safe(). (ie avoid to use
> per-cpu stat jiffies). Then, I don't think we have livelock chance.
> Am I missing something?
>

I have no idea. I just referenced the description in aa4548403.
As I look code, zone_watermark_ok_safe works well if percpu_drift_mark
is set rightly. but if memory hotplug happens, zone->present_pages
would be changed so that it can affect wmarks. It means it can affect
percpu_drift_mark, I think.

My point is to write down the description clear.

-- 
Kind regards,
Minchan Kim
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