linux-kernel - Re: [PATCH] x86: bump default NR

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Date:	Tue, 12 May 2015 13:57:14 -0400
From:	Josh Boyer <jwboyer@...oraproject.org>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86 <x86@...nel.org>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: bump default NR_CPUS for 64-bit configuration

On Fri, May 8, 2015 at 6:25 AM, Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
> Default NR_CPUS==8 is not enough to cover high-end desktop
> configuration: Haswell-E has upto 16 threads.
>
> Let's increase default NR_CPUS to 64 on 64-bit configuration. With this
> value CPU bitmask will still fit into one unsinged long.
>
> Default for 32-bit configuration is still 8: it's unlikely anybody will
> run 32-bit kernel on modern hardware.
>
> As alternative we could bump NR_CPUS to 128 to cover all dual-processor
> servers with some margin.
>
> For reference: Debian and Suse build their kernels with NR_CPUS==512,
> Fedora -- 1024.

FWIW, we're carrying a patch that drops the "if DEBUG_PER_CPU_MAPS"
dependency for CPUMASK_OFFSTACK to allow us to set that to 1024
without bringing in debug junk.  I sent this patch a long time ago and
Ingo and Linus said it was stupid and it should be auto-selected.
Ingo was going to poke at it IIRC how the thread ended, because I
couldn't see a clean way to make Kconfig do what we wanted.

josh

>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  arch/x86/Kconfig | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 6aaf38ac417d..c3333e5be5d7 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -851,7 +851,8 @@ config NR_CPUS
>         default "1" if !SMP
>         default "8192" if MAXSMP
>         default "32" if SMP && X86_BIGSMP
> -       default "8" if SMP
> +       default "8" if SMP && X86_32
> +       default "64" if SMP
>         ---help---
>           This allows you to specify the maximum number of CPUs which this
>           kernel will support.  If CPUMASK_OFFSTACK is enabled, the maximum
> --
> 2.1.4
>
> --
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