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SubjectRe: [PATCH 2/2] x86: Modify Kconfig to allow up to 4096 cpus
Adrian Bunk wrote:
> On Tue, Mar 25, 2008 at 06:41:39PM -0700, Mike Travis wrote:
>> Increase the limit of NR_CPUS to 4096 and introduce a boolean
>> called "MAXSMP" which when set (e.g. "allyesconfig"), will set
>> NR_CPUS = 4096 and NODES_SHIFT = 9 (512).
>
>
> I'm not really getting the point of MAXSMP - people should simply pick
> their values, and when they want the maximum "(2-4096)" and "(1-15)"
> already provide this information (except that your patch hides the
> latter information from the user).
>
> And with your patch, even with MAXSMP=y people could still set
> NR_CPUS=7 and NODES_SHIFT=15 or whatever else they want...
>
> More interesting would be why you want it to set NODES_SHIFT to
> something less than the maximum value of 15. I'm getting the fact that
> 2^15 > 4096 and that 15 might be nonsensical high, but this sounds more
> like requiring a patch to limit the range to 9?

I guess the main effect is that "MAXSMP" represents what's really
usable for an architecture based on other factors. The limit of
NODES_SHIFT = 15 is that it's represented in some places as a signed
16-bit value, so 15 is the hard limit without coding changes, not
an architecture limit.

Thanks,
Mike

>
>
>> ...
>> --- linux.trees.git.orig/arch/x86/Kconfig
>> +++ linux.trees.git/arch/x86/Kconfig
>> @@ -522,16 +522,24 @@ config SWIOTLB
>> access 32-bits of memory can be used on systems with more than
>> 3 GB of memory. If unsure, say Y.
>>
>> +config MAXSMP
>> + bool "Configure Maximum number of SMP Processors"
>> + depends on X86_64 && SMP
>> + default n
>> + help
>> + Configure maximum number of CPUS for this architecture.
>> + If unsure, say N.
>>
>> config NR_CPUS
>> - int "Maximum number of CPUs (2-255)"
>> - range 2 255
>> + int "Maximum number of CPUs (2-4096)"
>> + range 2 4096
>> depends on SMP
>> + default "4096" if MAXSMP
>> default "32" if X86_NUMAQ || X86_SUMMIT || X86_BIGSMP || X86_ES7000
>> default "8"
>> help
>> This allows you to specify the maximum number of CPUs which this
>> - kernel will support. The maximum supported value is 255 and the
>> + kernel will support. The maximum supported value is 4096 and the
>> minimum value which makes sense is 2.
>>
>> This is purely to save memory - each supported CPU adds
>> @@ -918,12 +926,16 @@ config NUMA_EMU
>> number of nodes. This is only useful for debugging.
>>
>> config NODES_SHIFT
>> - int "Max num nodes shift(1-15)"
>> + int "Maximum NUMA Nodes (as a power of 2)"
>> range 1 15 if X86_64
>> + default "9" if MAXSMP
>> default "6" if X86_64
>> default "4" if X86_NUMAQ
>> default "3"
>> depends on NEED_MULTIPLE_NODES
>> + help
>> + Specify the maximum number of NUMA Nodes available on the target
>> + system. Increases memory reserved to accomodate various tables.
>>
>> config HAVE_ARCH_BOOTMEM_NODE
>> def_bool y
>>
>
> cu
> Adrian
>



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