Subject: Re: [PATCH 1/1] hotplug cpu: migrate a task within its cpuset
From: Rusty Russell <rusty@rustcorp.com.au>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: ego@in.ibm.com, Oleg Nesterov <oleg@tv-sign.ru>,
       Cliff Wickman <cpw@sgi.com>, mingo@elte.hu, vatsa@in.ibm.com,
       pj@sgi.com, linux-kernel@vger.kernel.org, ntl@in.ibm.com,
       Dipankar Sarma <dipankar@in.ibm.com>
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Date: Mon, 27 Aug 2007 17:01:34 +1000
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On Sun, 2007-08-26 at 01:09 -0700, Andrew Morton wrote:
> On Sun, 26 Aug 2007 10:47:24 +1000 Rusty Russell <rusty@rustcorp.com.au> wrote:
> > 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.
> 
> That's a kernel bug.

You mean "would be if it were implemented"?  Although consider the
equivalent forkbomb or thrashing userspace problems, where we just say
"use quotas".

Just to clarify: that is not how we work, we migrate tasks off a dying
CPU, breaking affinity and printing a warning if necessary.  

It was simple and has not proven problematic in practice.  (Userspace
cpu affinity has been a question of optimality not correctness)

> > You could, perhaps, introduce a "gentle" offline which fails if process
> > affinity can no longer be met.
> 
> Suitably privileged userspace should be able to
> 
> 1) prevent tasks from binding to CPU N then
> 2) migrate all tasks which can use CPU N over to other CPU(s) then
> 3) offline CPU N.

Indeed, (1) is missing.  I would hesitate to introduce more
infrastructure in an under-worn and over-buggy part of the kernel
though.

Rusty.


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