Re: [PATCH] x86: Allow NR_CPUS=1024

[Josh Boyer <jwboyer@redhat.com>]

On Sun, Nov 03, 2013 at 11:21:32AM +0100, Ingo Molnar wrote:
> 
> * 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.

No real reason to not allow all the way to 4096, no.  I just started
small as I wanted 1024 specifically, and this is the simplest way to
achieve that.

> 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.

OK, that makes sense.  So in this scenario, we could probably either:

a) do away with MAXSMP entirely and just depend on
CONFIG_CPUMASK_OFFSTACK.

b) make MAXSMP something even higher than 4096.  Like 5120 or 6144, etc.

Which would you prefer?  Either is easy enough to code up, I just need
to know which I should shoot for.

josh