linux-kernel - Re: Inquiry: Should we remove "isolcpus= kernel boot option? (may have realtime uses)

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Message-ID: <48466745.5050802@cfl.rr.com>
Date:	Wed, 04 Jun 2008 05:58:29 -0400
From:	Mark Hounschell <dmarkh@....rr.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Oeser <ioe-lkml@...eria.de>, Paul Jackson <pj@....com>,
	linux-kernel@...r.kernel.org, Con Kolivas <kernel@...ivas.org>,
	"Derek L. Fults" <dfults@....com>, devik <devik@....cz>,
	Dimitri Sivanich <sivanich@....com>,
	Dinakar Guniguntala <dino@...ibm.com>,
	Emmanuel Pacaud <emmanuel.pacaud@...v-poitiers.fr>,
	Frederik Deweerdt <deweerdt@...e.fr>,
	Ingo Molnar <mingo@...e.hu>,
	Matthew Dobson <colpatch@...ibm.com>,
	Max Krasnyansky <maxk@...lcomm.com>, rostedt@...dmis.org,
	Oleg Nesterov <oleg@...sign.ru>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Paul Menage <menage@...gle.com>,
	"Randy.Dunlap" <rddunlap@...l.org>, suresh.b.siddha@...el.com,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Inquiry: Should we remove "isolcpus= kernel boot option? (may
 have realtime uses)

Nick Piggin wrote:
> On Tuesday 03 June 2008 08:45, Peter Zijlstra wrote:
>> On Tue, 2008-06-03 at 00:35 +0200, Ingo Oeser wrote:
>>> Hi Paul,
>>>
>>> in short: NAK!
>>>
>>> On Monday 02 June 2008, Paul Jackson wrote:
>>>> (Aside to the RealTime folks -- is there a 'realtime'
>>>> email list which I should include in this discussion?)
>>>>
>>>> The kernel has a "isolcpus=" kernel boot time parameter.  This
>>>> parameter isolates CPUs from scheduler load balancing, minimizing the
>>>> impact of scheduler latencies on realtime tasks running on those CPUs.
>>> I used it to mask out a defect CPU on a 8-CPU node of a
>>> HPC-cluster at a customer site, until the $BIG_VENDOR
>>> sent a replacement. And to prove $BIG_VENDOR, that we actually
>>> have a problem on THAT CPU.
>>>
>>> So I would really like to keep this fault isolation capability.
>>> I made my customer happy with that.
>>>
>>> I wish Linux had more such "mask out bad hardware" features
>>> to faciliate fault isolation and boot and runtime.
>> Yeah - except that its not meant to be used as such - it will still
>> brings the cpu up, and it is still usable for the OS.
>>
>> So sorry, your abuse doesn't make for a case to keep this abomination.
> 
> How come it is an abonination? It is an easy way to do what it does,
> and it's actually not a bad thing for some uses not to have to use
> cpusets.
> 
> Given that it's all __init code anyway, is there a real reason _to_
> remove it?

IMHO,

What is an abonination, is that cpusets are equired for this type of 
isolation to begin with, even on a 2 processor machine.

I would like the option to stay and be extended like Max originally
proposed. If cpusets/hotplug are configured isolation would be obtained 
using them. If not then isolcpus could be used to get the same isolation.

 From a user land point of view, I just want an easy way to fully 
isolate a particular cpu. Even a new syscall or extension to 
sched_setaffinity would make me happy. Cpusets and hotplug don't.

Again this is just MHO.

Regards
Mark

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