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

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Message-ID: <86802c441001121157y5cdc1a91g2a66d8591d6fd579@mail.gmail.com>
Date:	Tue, 12 Jan 2010 11:57:38 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Christoph Lameter <cl@...ux-foundation.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, Jan 12, 2010 at 11:32 AM, Christoph Lameter
<cl@...ux-foundation.org> wrote:
>
> On Tue, 12 Jan 2010, Yinghai Lu wrote:
>
>> on x86, before prefill_possible_map(), nr_cpu_ids will be NR_CPUS aka CONFIG_NR_CPUS
>>
>> add nr_cpus= to set nr_cpu_ids. so we can simulate cpus <=8 on normal config.
>> instead of change NR_CPUS directly.
>
> We already have
>
> maxcpus=x?
>
> 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.

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.

YH
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