linux-kernel - Re: [RFC PATCH 1/5] use nr_cpus= to set nr_cpu

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Date:	Tue, 12 Jan 2010 14:31:53 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Yinghai Lu <yinghai@...nel.org>
cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"ananth@...ibm.com" <ananth@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/5] use nr_cpus= to set nr_cpu_ids early

On Tue, 12 Jan 2010, Yinghai Lu wrote:

> > Why do we need this twice?
>
> maxcpus only change setup_max_cpus., and if you are using maxcpus=1,
> and you have 8 cpus installed, you can put other
> cpus back online via /sys/interface.

Hmmm.. Strange semantics since maxcpus=0 disables smp completely. No cpu
can be activated later. Similar to nr_cpus ?

> nr_cpus= is hard limit nr_cpu_ids, so if you have 16 cpus installed,
> nr_cpus=8 will make your nr_cpu_ids=8, and you can not put back
> other 8 back. and apic mode could stay with logical flat.
> this is used to simulate some debug case. for example you have kernel
> support physflat, and flat, with CONFIG_NR_CPUS=255. to run that on
> system that only have 8 cpus, you will have apic mode in logical flat.
>  but if you have run the kernel on system with 32 cpus installed, it
> will switch to physflat even you have maxcpus=8 appedded.

Ok makes sense.
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