linux-kernel - Re: [PATCH] add command line init_start

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Date:	Tue, 16 Dec 2008 22:18:48 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] add command line init_start_cpus

* Yinghai Lu <yinghai@...nel.org> wrote:

> On Sun, Dec 14, 2008 at 4:11 PM, Andi Kleen <andi@...stfloor.org> wrote:
> > Yinghai Lu <yinghai@...nel.org> writes:
> >
> >> Impact: new command line
> >>
> >> so could select cpus to be started during init stage
> >
> > maxcpus=N should do this already, at least for the first N cpus.
> > If you really need a non continuous range of N (why?) then it would
> > be better to fix maxcpus instead of adding a new option.
> 
> it seems it is hard to make maxcpus= to support random cpus selecting 
> and maxcpus=0 looks insane

yes, maxcpus=0 looks sucky. If we want this via a boot option then it 
should be a separate one.

> also maxcpus=1, will disable smp, can you enable other cpus later in 
> user space?

only booting maxcpus=0 or nosmp will disable SMP permanently. maxcpus=1 
should give the boot CPU [if not then we need to fix it], and you can then 
enable an arbitrary permutation of good CPUs via:

  echo 1 > /sys/devices/system/cpu/cpuX/online

as long as you CONFIG_HOTPLUG_CPU enabled.

What _might_ make sense is to introduce badcpus=2,4,6 that would 
permanently disnumerate all the broken CPUs, by striking that mask from 
the possible-cpus-map - inaccessible to hotplugging as well. (Note that by 
naming it in the inverse way the thing gets a whole new quality - the boot 
option in itself explains its purpose.)

	Ingo
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