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

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Date:	Mon, 02 Jun 2008 16:04:21 -0700
From:	Max Krasnyansky <maxk@...lcomm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	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>,
	Nick Piggin <nickpiggin@...oo.com.au>, 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)

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.

Ingo, I just wanted to elaborate on what Peter is saying. That CPU will still
have to be _booted_ properly. It may be used for hard- and soft- interrupt
processing, workqueues (internal kernel queuing mechanism) and kernel timers.

In your particular case you're much much much better off with doing
	echo 0 > /sys/devices/system/cpuN/online
either during initrd stage or as a first init script.
That way bad cpu will be _completely_ disabled.

Max
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