linux-kernel - Re: 2.6.27rc1 cannot boot more than 8CPUs

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Message-ID: <20080813173940.GA32214@elte.hu>
Date:	Wed, 13 Aug 2008 19:39:40 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeff Chua <jeff.chua.linux@...il.com>
Cc:	Yinghai Lu <yhlu.kernel@...il.com>,
	David Miller <davem@...emloft.net>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.27rc1 cannot boot more than 8CPUs


* Jeff Chua <jeff.chua.linux@...il.com> wrote:

> On Thu, Aug 14, 2008 at 1:10 AM, Jeff Chua <jeff.chua.linux@...il.com> wrote:
> > On Wed, Aug 13, 2008 at 10:16 PM, Ingo Molnar <mingo@...e.hu> wrote:
> >
> >> btw., could you please check that v2.6.27-rc3 (or later) kernels boot
> >> fine (with about 8 cpus) even if you have genericarch/bigsmp disabled,
> >> and do not silently hang as it happened on your box before?
> >
> > With 16 CPUs, it still hangs, but now the console is showing the
> > errors as intended.
> > ... but it is supposed to hang?
> 
> I tried with just CONFIG_NR_CPUS=8 and this time it booted, but stange
> thing is I only see 2 CPUs! To be more precise, it's without both
> CONFIG_X86_GENERICARCH and CONFIG_X86_BIGSMP.
> 
> And when I tried to enable the CPUs, it complained about:
> 
> # cat cpu6/online
> 0
> # echo 1 > cpu6/online
> More than 8 CPUs detected - skipping them.
> Use CONFIG_X86_GENERICARCH and CONFIG_X86_BIGSMP.
> -bash: echo: write error: Input/output error
> 
> Prior to the patch, the system booted with all 8 CPUs.
> 
> Again, if I enable both CONFIG_X86_GENERICARCH and CONFIG_X86_BIGSMP,
> I get all 16 CPUs.

Yinghai, could the APIC ID enumeration be nonsequential and we skip CPUs 
starting at the third one already? I think we should accept all CPUs 
that are within our support range.

	Ingo
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