Re: [PATCH] arm64: Kconfig: Don't select CPUMASK_OFFSTACK with 64K pages

[Breno Leitao <leitao@debian.org>]

On Wed, Oct 07, 2026 at 12:58:16AM -0700, Breno Leitao wrote:
> arm64 selects CPUMASK_OFFSTACK when NR_CPUS is above 256, which keeps
> cpumask_var_t off the kernel stack. With 64K pages the kernel stack is
> 64K as well, we have enough room to keep cpumask on the stack, avoiding
> extra alocation and possible performance penalties.
> 
> Skip the select when ARM64_64K_PAGES is set. A 64K kernel can then raise
> NR_CPUS above 256 and keep cpumask_var_t as an inline array. 4K and 16K
> page kernels are unchanged.
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
> Based on Will presentation at LPC, I've decided to have a look at Meta's
> stack usage, and I found that this might be an small optimization!?
> ---
>  arch/arm64/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 5f2db2cecf9772..b73371327d2a47 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -111,7 +111,7 @@ config ARM64
>  	select CLONE_BACKWARDS
>  	select COMMON_CLK
>  	select CPU_PM if (SUSPEND || CPU_IDLE)
> -	select CPUMASK_OFFSTACK if NR_CPUS > 256
> +	select CPUMASK_OFFSTACK if NR_CPUS > 256 && !ARM64_64K_PAGES

Does this one sound sane?