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Date:	Sun, 3 Nov 2013 11:21:32 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Josh Boyer <jwboyer@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, prarit@...hat.com,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Allow NR_CPUS=1024


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Josh Boyer <jwboyer@...hat.com> wrote:
> 
> > The current range for SMP configs is 2 - 512, or a full 4096 in the case 
> > of MAXSMP.  There are machines that have 1024 CPUs in them today and 
> > configuring a kernel for that means you are forced to set MAXSMP.  This 
> > adds additional unnecessary overhead.  While that overhead might be 
> > considered tiny for large machines, it isn't necessarily so if you are 
> > building a kernel that runs across a wide variety of machines.  We 
> > increase the range to 1024 to help with this.
> >
> > Signed-off-by: Josh Boyer <jwboyer@...oraproject.org>
> > ---
> >  arch/x86/Kconfig | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index f67e839..d726b2d 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -825,7 +825,7 @@ config MAXSMP
> >  config NR_CPUS
> >  	int "Maximum number of CPUs" if SMP && !MAXSMP
> >  	range 2 8 if SMP && X86_32 && !X86_BIGSMP
> > -	range 2 512 if SMP && !MAXSMP
> > +	range 2 1024 if SMP && !MAXSMP
> >  	default "1" if !SMP
> >  	default "4096" if MAXSMP
> >  	default "32" if SMP && (X86_NUMAQ || X86_SUMMIT || X86_BIGSMP || X86_ES7000)
> 
> Any reason not to allow it to go up to 4096? The original concern was 
> that CPUS=4096 wasn't working very well and you had to select MAXSMP 
> deliberately and keep all the pieces.

The other reason was CONFIG_CPUMASK_OFFSTACK: with 4096 CPUs a cpumask is 
512 bytes, too large to be kept on the kernel stack.

MAXSMP forces CONFIG_CPUMASK_OFFSTACK so there's no such concern there.

With 1024 CPUs a single cpumask is 128 bytes - rather significant as well. 
With 512 CPUs it's 64 bytes - borderline.

So I think a better solution would be to allow an increase above 512 CPUs 
only if CONFIG_CPUMASK_OFFSTACK is also enabled.

Thanks,

	Ingo
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