linux-kernel - Re: [PATCH 0/3] mm: fixup changers of per cpu pageset's ->high and ->batch

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Message-ID: <5162FB82.5020607@linux.vnet.ibm.com>
Date:	Mon, 08 Apr 2013 10:16:50 -0700
From:	Cody P Schafer <cody@...ux.vnet.ibm.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] mm: fixup changers of per cpu pageset's ->high and
 ->batch

On 04/07/2013 08:23 AM, KOSAKI Motohiro wrote:
> (4/5/13 4:33 PM), Cody P Schafer wrote:
>> In one case while modifying the ->high and ->batch fields of per cpu pagesets
>> we're unneededly using stop_machine() (patches 1 & 2), and in another we don't have any
>> syncronization at all (patch 3).
>>
>> This patchset fixes both of them.
>>
>> Note that it results in a change to the behavior of zone_pcp_update(), which is
>> used by memory_hotplug. I _think_ that I've diserned (and preserved) the
>> essential behavior (changing ->high and ->batch), and only eliminated unneeded
>> actions (draining the per cpu pages), but this may not be the case.
> 
> at least, memory hotplug need to drain.

Could you explain why the drain is required here? From what I can tell,
after the stop_machine() completes, the per cpu page sets could be
repopulated at any point, making the combination of draining and
modifying ->batch & ->high uneeded.

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