Subject: Re: Inquiry: Should we remove "isolcpus= kernel boot option? (may
	have realtime uses)
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Max Krasnyansky <maxk@qualcomm.com>
Cc: Paul Jackson <pj@sgi.com>, Andi Kleen <andi@firstfloor.org>,
       ioe-lkml@rameria.de, sivanich@sgi.com, linux-kernel@vger.kernel.org,
       kernel@kolivas.org, dfults@sgi.com, devik@cdi.cz, dino@in.ibm.com,
       emmanuel.pacaud@univ-poitiers.fr, deweerdt@free.fr, mingo@elte.hu,
       colpatch@us.ibm.com, nickpiggin@yahoo.com.au, rostedt@goodmis.org,
       oleg@tv-sign.ru, paulmck@us.ibm.com, menage@google.com,
       rddunlap@osdl.org, suresh.b.siddha@intel.com, tglx@linutronix.de
In-Reply-To: <4846DF25.4020300@qualcomm.com>
References: <20080601213019.14ea8ef8.pj@sgi.com>
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	 <48447C75.8040203@qualcomm.com>	<200806030156.00155.ioe-lkml@rameria.de>
	 <4844BB41.7000605@qualcomm.com>	<4845D825.6000403@qualcomm.com>
	 <20080603194148.56dfebe1.pj@sgi.com>	<48461ADA.8020303@qualcomm.com>
	 <877id5v1fg.fsf@basil.nowhere.org> <20080604124126.0d281a99.pj@sgi.com>
	 <4846DF25.4020300@qualcomm.com>
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Date: Wed, 04 Jun 2008 20:56:34 +0200
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On Wed, 2008-06-04 at 11:29 -0700, Max Krasnyansky wrote:
> 
> Paul Jackson wrote:
> > Andi wrote:
> >> Right now the system boot could put pages from some daemon in there before any 
> >> cpusets are set up and there's no easy way to get them away again
> > 
> > We (SGI) routinely handle that need with a custom init program,
> > invoked with the init= parameter to the booting kernel, which
> > sets up cpusets and then invokes the normal (real) init program
> > in a cpuset configured to exclude those CPUs and nodes which we
> > want to remain unloaded.  For example, on a 256 CPU, 64 node
> > system, we might have init running on a single node of 4 CPUs,
> > and leave the remaining 63 nodes and 252 CPUs isolated from all
> > the usual user level daemons started by init.
> > 
> > There is no need for additional kernel changes to accomplish this.
> 
> You do not even need to replace /sbin/init for this, no ?
> Simply installing custom
> 	/etc/init.d/create_cpusets
> with priority 0
> 	# chkconfig: 12345 0 99
> will do the job.
> 
> That script will move init itself into the appropriate cpuset and from then on
> everything will inherit it.

The advantage of using a replacement /sbin/init is that you execute
before the rest of userspace, unlike what you propose.

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