Re: [PATCH] x86/Kconfig: Allow NR_CPUS between 512 and 8192

[Saurabh Singh Sengar]

On Tue, Feb 20, 2024 at 01:50:13AM -0800, Saurabh Sengar wrote:
> Today there is no way one can choose any value between 512 to 8192
> for NR_CPUS seamlessly. NR_CPUS is guarded by NR_CPUS_RANGE_END which
> is further dependent on CPUMASK_OFFSTACK to allow NR_CPUs > 512.
> 
> For x86, CPUMASK_OFFSTACK can only be enabled either by selecting MAXSMP
> or DEBUG_PER_CPU_MAPS. Both of these options has a cost to pay. MAXSMP
> will increase the NR_CPUS to 8192 which will have impact on kernel image
> size whereas DEBUG_PER_CPU_MAPS will have additional run time overheads.
> Thus there is no good way to have NR_CPUS anything between 512 to 8192.
> 
> Fix this by selecting CPUMASK_OFFSTACK if NR_CPUS > 512 and
> let NR_CPUS_RANGE_END set to 8192.
> 
> On a Hyper-V system where max number of CPUs are only 2048, this
> patch saves around 1 MB of kernel image size, compare to MAXSMP.
> 
> Signed-off-by: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx>
> ---
> 
> I want to mention that in ARM and other archs its very simple
> to select any value for NR_CPUS. This is an attempt to have more
> flexibilty in x86 arch as well to choose NR_CPUS.
> 
> Some of the earlier discussions reated to it which could be of interest:
> https://lore.kernel.org/lkml/1708092603-14504-1-git-send-email-ssengar@xxxxxxxxxxxxxxxxxxx/
> https://lore.kernel.org/lkml/794a1211-630b-3ee5-55a3-c06f10df1490@xxxxxxxxx/
> 
> Another approach I can think of is to allow CPUMASK_OFFSTACK to be enabled
> more freely like the below patch of Libo Chen, that will also solve the
> problem I am addressing. But I feel this patch may have impact on other
> archs as well and I am not sure if that is in best interest of all the archs.
> 
> https://lore.kernel.org/lkml/20220412231508.32629-2-libo.chen@xxxxxxxxxx/
> 
>  arch/x86/Kconfig | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 07a0c8d4e9c7..458f3f250d7f 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -34,6 +34,7 @@ config X86_64
>  	select SWIOTLB
>  	select ARCH_HAS_ELFCORE_COMPAT
>  	select ZONE_DMA32
> +	select CPUMASK_OFFSTACK if NR_CPUS > 512
>  
>  config FORCE_DYNAMIC_FTRACE
>  	def_bool y
> @@ -1006,8 +1007,7 @@ config NR_CPUS_RANGE_END
>  config NR_CPUS_RANGE_END
>  	int
>  	depends on X86_64
> -	default 8192 if  SMP && CPUMASK_OFFSTACK
> -	default  512 if  SMP && !CPUMASK_OFFSTACK
> +	default 8192 if  SMP
>  	default    1 if !SMP
>  
>  config NR_CPUS_DEFAULT
> 
> -- 
> 2.34.1
> 

x86 Maintainers,

Kind reminder to have your feedback on this patch.

- Saurabh