linux-kernel - Re: RFC: THE OFFLINE SCHEDULER

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Message-ID: <84144f020908260821n5b0b9f7ejec8bbe9586150f37@mail.gmail.com>
Date:	Wed, 26 Aug 2009 18:21:53 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Chris Friesen <cfriesen@...tel.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	raz ben yehuda <raziebe@...il.com>, riel@...hat.com,
	mingo@...e.hu, andrew motron <akpm@...ux-foundation.org>,
	wiseman@...s.biu.ac.il, lkml <linux-kernel@...r.kernel.org>,
	linux-rt-users@...r.kernel.org
Subject: Re: RFC: THE OFFLINE SCHEDULER

Hi Peter,

On Wed, Aug 26, 2009 at 8:31 AM, Peter Zijlstra<peterz@...radead.org> wrote:
>> Is it the whole concept of isolating one or more cpus from all normal
>> kernel tasks that you don't like, or just this particular implementation?
>>
>> I ask because I know of at least one project that would have used this
>> capability had it been available.  As it stands they have to live with
>> the usual kernel threads running on the cpu that they're trying to
>> dedicate to their app.
>
> Its the simple fact of going around the kernel instead of using the
> kernel.
>
> Going around the kernel doesn't benefit anybody, least of all Linux.
>
> So its the concept of running stuff on a CPU outside of Linux that I
> don't like. I mean, if you want that, go ahead and run RTLinux, RTAI,
> L4-Linux etc.. lots of special non-Linux hypervisor/exo-kernel like
> things around for you to run things outside Linux with.

Out of curiosity, what's the problem with it? Why can't the scheduler
be taught to bind one user-space thread on a given CPU and make sure
no other threads are scheduled on that CPU? I'm not a scheduler expert
but that seems like a logical extension to the current cpuset logic
and would help the low-latency workload Christoph has described in the
past.

                        Pekka
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