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Date: Mon, 4 Nov 2013 07:53:43 +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 * Josh Boyer <jwboyer@...hat.com> wrote: > On Sun, Nov 03, 2013 at 11:21:32AM +0100, Ingo Molnar wrote: > > > > * 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. > > 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. Why touch MAXSMP at all? It's really just a shortcut for 'configure the kernel silly large', via a single option, nothing else. You are not forced to use it and it should not affect configurability of NR_CPUS. What we _really_ want here is to fix NR_CPUS setting: to extend its range and to enforce that NR_CPUS cannot be set larger than 512 without setting CONFIG_CPUMASK_OFFSTACK. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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