linux-kernel - Re: [PATCH 05/10] cpuset: don't update tasks' cpumask and nodemask in an empty cpuset

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Date:	Sat, 20 Apr 2013 09:06:44 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Tejun Heo <tj@...nel.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>,
	Containers <containers@...ts.linux-foundation.org>,
	Glauber Costa <glommer@...allels.com>
Subject: Re: [PATCH 05/10] cpuset: don't update tasks' cpumask and nodemask
 in an empty cpuset

On 2013/4/20 2:36, Tejun Heo wrote:
> Hello, Li.
> 
> On Fri, Apr 19, 2013 at 08:27:05PM +0800, Li Zefan wrote:
>> I think this was introduced unintentionally when cpuset hotplug was
>> made asynchronous. Fortunately it does no harm, as updating tasks'
>> cpumask will just return failure and there's a guarantee_online_mems()
>> when updating nodemask, and then the tasks will be moved to an ancestor
>> cpuset.
> 
> Yeah, which will update the masks to the proper values anyway, so it
> was intentionally written that way as it didn't really matter either
> way.  I suppose this helps future changes?  Maybe update the
> description a bit?
> 

I was trying to fix a bug: before moving tasks out of empty cpusets,
update_tasks_nodemask() is called, which calls do_migrate_pages(xx, from, to)
finally, from == node_to_be_offlined, to == empty_nodeamsk, so I guess
no pages will be migrated.

Then when those tasks are moving to an ancestor, do_migrate_pages(xx, from, to)
is called again, from == empty_nodemask, to == ancestor's nodemask,
so I guess again no pages will be migrated.

This bug exists even before your changes.

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