linux-kernel - Re: [PATCH 1/1] hotplug cpu: migrate a task within its cpuset

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Message-ID: <20070315003642.GB7338@lnx-holt.americas.sgi.com>
Date:	Wed, 14 Mar 2007 19:36:43 -0500
From:	Robin Holt <holt@....com>
To:	Nathan Lynch <ntl@...ox.com>
Cc:	Cliff Wickman <cpw@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] hotplug cpu: migrate a task within its cpuset

On Fri, Mar 09, 2007 at 05:58:59PM -0600, Nathan Lynch wrote:
> Hello-
> 
> Cliff Wickman wrote:
> > This patch would insert a preference to migrate such a task to a cpu within
> > its cpuset (and set its cpus_allowed to its cpuset).
> > 
> > With this patch, migrate the task to:
> >  1) to any cpu on the same node as the disabled cpu, which is both online
> >     and among that task's cpus_allowed
> >  2) to any online cpu within the task's cpuset
> >  3) to any cpu which is both online and among that task's cpus_allowed
> 
> I think I disagree with this change.
> 
> The kernel shouldn't have to be any smarter than it already is about
> moving tasks off an offlined cpu.  The only way case 2) can be reached
> is if the user has changed a task's cpu affinity.  If the user is
> sophisticated enough to manipulate tasks' cpu affinity then they can
> arrange to migrate tasks as they see fit before offlining a cpu.

You are assuming some sort of interlock between the admin and the user.
While this may be true on your own personal desktop, I don't think you
can expect this to be true on a development machine shared by hundreds
of users and admin'd by a group of people.

Additionally, ia64 is gaining support for offlining a cpu which is giving
cache errors.

Thanks,
Robin
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