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

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Date:	Sun, 03 Nov 2013 09:29:16 -0500
From:	Prarit Bhargava <prarit@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Josh Boyer <jwboyer@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Russ Anderson <rja@....com>
Subject: Re: [PATCH] x86: Allow NR_CPUS=1024



On 11/03/2013 05:18 AM, Ingo Molnar 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.
> 
> But today it's all pretty robust so I see no reason why not to allow up to 
> 4096 CPUs.

Adding Russ from SGI as they are one of the consumers of a large CPU count.

I have no objections to raising this to 4096 FWIW.  I think it is a good idea,
and it is long overdue.

P.

> 
> Thanks,
> 
> 	Ingo
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