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

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Date:	Mon, 2 Jun 2008 16:41:51 -0500
From:	Dimitri Sivanich <sivanich@....com>
To:	Max Krasnyansky <maxk@...lcomm.com>
Cc:	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>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"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)

On Mon, Jun 02, 2008 at 11:39:34AM -0700, Max Krasnyansky wrote:
> Ah, I know exactly what you're talking about.  However this is non-issue these
> days. In order to clear cpuN from all the timers and other things all you need
> to do is to bring that cpu off-line
> 	echo 0 > /sys/devices/cpu/cpuN/online
> and then bring it back online
> 	echo 1 > /sys/devices/cpu/cpuN/online

Although it seemed like something of a hack, we experimented with this
previously and found that it didn't work reliably.  I'm sure things
have gotten better, but will need to revisit.

> 
> There are currently a couple of issues with scheduler domains and hotplug
> event handling. I do have the fix for them, and Paul had already acked it.

Until a proven reliable method for doing this is firmly in place (as
firmly as anything is, anyway),  I don't think we should be removing
the alternative.

> initialization). See my latest "default IRQ affinity" patch.

Nice idea.

> Also isolcpus= conflicts with the scheduler domains created by the cpusets.

What sort of conflict are we talking about?  I assume once you've begun setting up cpusets that include those cpus that you're intention is to change the original behavior.

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