linux-kernel - Re: [PATCH -mm] do_migrate_pages() calls migrate_to_node() even if task is already on a correct node

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Date:	Thu, 22 Mar 2012 14:47:09 -0400
From:	Rik van Riel <riel@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
CC:	lwoodman@...hat.com, Christoph Lameter <cl@...ux.com>,
	linux-mm@...ck.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Motohiro Kosaki <mkosaki@...hat.com>
Subject: Re: [PATCH -mm] do_migrate_pages() calls migrate_to_node() even if
 task is already on a correct node

On 03/22/2012 02:45 PM, KOSAKI Motohiro wrote:
> CC to Christoph.
>
>> While moving tasks between cpusets I noticed some strange behavior.
>> Specifically if the nodes of the destination
>> cpuset are a subset of the nodes of the source cpuset
>> do_migrate_pages() will move pages that are already on a node
>> in the destination cpuset. The reason for this is do_migrate_pages()
>> does not check whether each node in the source
>> nodemask is in the destination nodemask before calling
>> migrate_to_node(). If we simply do this check and skip them
>> when the source is in the destination moving we wont move nodes that
>> dont need to be moved.
>>
>> Adding a little debug printk to migrate_to_node():
>>
>> Without this change migrating tasks from a cpuset containing nodes 0-7
>> to a cpuset containing nodes 3-4, we migrate
>> from ALL the nodes even if they are in the both the source and
>> destination nodesets:
>>
>> Migrating 7 to 4
>> Migrating 6 to 3
>> Migrating 5 to 4
>> Migrating 4 to 3
>> Migrating 1 to 4
>> Migrating 3 to 4
>> Migrating 0 to 3
>> Migrating 2 to 3
>
> Wait.
>
> This may be non-optimal for cpusets, but maybe optimal migrate_pages,
> especially
> the usecase is HPC. I guess this is intended behavior. I think we need
> to hear
> Christoph's intention.

How is the moving of pages that are already in the
intended cpuset better than just leaving them alone?

> But, I'm not against this if he has no objection.

Sure. Christoph?
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