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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Date:	Mon, 08 Apr 2013 15:26:55 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Cody P Schafer <cody@...ux.vnet.ibm.com>
CC:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	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()

(4/8/13 1:32 PM), Cody P Schafer wrote:
> 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).

maybe yes, maybe no. I don't know temporary or not. However the fact is, 
you must not break anywhere. You need to look all caller always.


>> 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.

OK, I missed that. however your code is still wrong.
However you can't call free_pcppages_bulk() from interrupt context and
then you can't use on_each_cpu() anyway.






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