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

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Message-Id: <1188089244.19877.3.camel@localhost.localdomain>
Date:	Sun, 26 Aug 2007 10:47:24 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	ego@...ibm.com
Cc:	Oleg Nesterov <oleg@...sign.ru>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cliff Wickman <cpw@....com>, mingo@...e.hu, vatsa@...ibm.com,
	pj@....com, linux-kernel@...r.kernel.org, ntl@...ibm.com,
	Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH 1/1] hotplug cpu: migrate a task within its cpuset

On Sun, 2007-08-26 at 05:46 +0530, Gautham R Shenoy wrote:
> On Sat, Aug 25, 2007 at 01:47:40PM +0400, Oleg Nesterov wrote:
> > Before this patch, process leaves its ->cpuset and migrates to some "random"
> > any_online_cpu(). With this patch it stays within ->cpuset and migrates to
> > CPU 3.
> 
> The decision to bind a task to a specific cpu, was taken by the userspace
> for a reason, which is _unknown_ to the kernel.
> So logically, shouldn't the userspace decide what should be 
> the fate of those exclusive-affined tasks, whose cpu is about to go
> offline? After all, the reason to offline the cpu is, again, unknown to
> the kernel.

Userspace is not monolithic.  If you refuse to take a CPU offline
because a task is affine, then any user can prevent a CPU from going
offline.

You could, perhaps, introduce a "gentle" offline which fails if process
affinity can no longer be met.

Rusty.

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