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

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Message-ID: <516333F2.7090706@linux.vnet.ibm.com>
Date:	Mon, 08 Apr 2013 14:17:38 -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 04/08/2013 12:08 PM, KOSAKI Motohiro wrote:
> (4/8/13 1:16 PM), Cody P Schafer wrote:
>> 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.
> 
> Then, memory hotplug again and again try to drain. Moreover hotplug prevent repopulation
> by using MIGRATE_ISOLATE.
> pcp never be page count == 0 and it prevent memory hot remove.

zone_pcp_update() is not part of the drain retry loop, in the offline
pages case, it is only called following success in "removal" (online
pages also calls zone_pcp_update(), I don't see how it could benifit in
any way from draining the per cpu pagesets).

So I still don't see where the need for draining pages combined with
modifying ->high and ->batch came from.

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