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SubjectRe: [PATCH] x86: Allow NR_CPUS=1024

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

>
> * Josh Boyer <jwboyer@redhat.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@fedoraproject.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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