linux-kernel - Re: [PATCH 2/3] mm/page_alloc: convert zone_pcp_update() to use on_each_cpu() instead of stop

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Message-ID: <5162FF18.8010802@linux.vnet.ibm.com>
Date:	Mon, 08 Apr 2013 10:32:08 -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 2/3] mm/page_alloc: convert zone_pcp_update() to use on_each_cpu()
 instead of stop_machine()

On 04/07/2013 08:39 AM, KOSAKI Motohiro wrote:
> (4/5/13 4:33 PM), Cody P Schafer wrote:
>> No off-cpu users of the percpu pagesets exist.
>>
>> zone_pcp_update()'s goal is to adjust the ->high and ->mark members of a
>> percpu pageset based on a zone's ->managed_pages. We don't need to drain
>> the entire percpu pageset just to modify these fields. Avoid calling
>> setup_pageset() (and the draining required to call it) and instead just
>> set the fields' values.
>>
>> This does change the behavior of zone_pcp_update() as the percpu
>> pagesets will not be drained when zone_pcp_update() is called (they will
>> end up being shrunk, not completely drained, later when a 0-order page
>> is freed in free_hot_cold_page()).
>>
>> Signed-off-by: Cody P Schafer <cody@...ux.vnet.ibm.com>
> 
> NAK.
> 
> 1) zone_pcp_update() is only used from memory hotplug and it require page drain.

I'm looking at this code because I'm currently working on a patchset
which adds another interface which modifies zone sizes, so "only used
from memory hotplug" is a temporary thing (unless I discover that
zone_pcp_update() is not intended to do what I want it to do).

> 2) stop_machin is used for avoiding race. just removing it is insane.

What race? Is there a cross cpu access to ->high & ->batch that makes
using on_each_cpu() instead of stop_machine() inappropriate? It is
absolutely not just being removed.

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