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: <51631F4D.7050504@linux.vnet.ibm.com>
Date:	Mon, 08 Apr 2013 12:49:33 -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 04/08/2013 12:26 PM, KOSAKI Motohiro wrote:
> (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.

Right, which is why I want to understand memory hotplug's actual 
requirements.

>>> 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.

Given drain_pages() implementation, I find that hard to believe (It uses 
on_each_cpu_mask() and eventually calls free_pcppages_bulk()).

Can you provide a reference backing up your statement?

If this turns out to be an issue, schedule_on_each_cpu() could be an 
alternative.

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