linux-kernel - Re: [PATCH] x86: Allow NR

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Message-ID: <20131103101825.GA6605@gmail.com>
Date:	Sun, 3 Nov 2013 11:18:25 +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:

> 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.

But today it's all pretty robust so I see no reason why not to allow up to 
4096 CPUs.

Thanks,

	Ingo
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