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

[Will Deacon <will@kernel.org>]

On Tue, Oct 27, 2026 at 04:59:52AM -0700, Breno Leitao wrote:
> 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?

It doesn't make an awful lot of sense to me, to be honest. The page size
and the number of CPUs are two independent variables, so tying them
together like this looks a little odd.

Will