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

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Date:	Tue, 3 Jun 2008 16:03:39 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
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>,
	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)

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?
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