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

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Date:	Tue, 03 Jun 2008 16:47:49 -0700
From:	Max Krasnyanskiy <maxk@...lcomm.com>
To:	Ingo Oeser <ioe-lkml@...eria.de>,
	Dimitri Sivanich <sivanich@....com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, Paul Jackson <pj@....com>,
	linux-kernel@...r.kernel.org, Con Kolivas <kernel@...ivas.org>,
	"Derek L. Fults" <dfults@....com>, devik <devik@....cz>,
	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)

>> I would really appreciate some way to keep the kernel from using 
>> a CPU at all to do fault isolation. If possible not even booting it.
> How does isolcpu= boot option helps in this case ?
> I suppose the closes option is maxcpus=. We can probably add ignorecpus= or
> something to handle your use case but it has nothing to do with isolcpus=.

btw Ingo, I just realized that maxcpu= option is exactly what you need.
Here is how you can use it.
Boot your system with maxcpus=1. That way the kernel will only bring up 
processor 0. I'm assuming cpu0 is "good" otherwise your system is totally 
busted :). Other cpus will stay off-line and will not be initialized.
Then once the system boots you can selectively bring "good" processors online 
by doing
	echo 1 > /sys/devices/system/cpu/cpuN/online

This actually solves the case you're talking about (ie ignoring bad 
processors) instead of partially covering it with isolcpus=.

Dimitri, you can probably use that too. ie Boot the thing with most CPUs 
offline and then bring them online. That way you'll know for sure that no 
timers, works, hard-/soft-irqs, etc are running on them.

So I expect two ACKs for isolcpu= removal from both of you, in bold please :)

Max
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