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

[Breno Leitao <leitao@debian.org>]

On Fri, Oct 30, 2026 at 11:53:56AM +0000, Will Deacon wrote:
> Because you might have thousands of CPUs?

I checked that case. I built defconfig with 64K pages and NR_CPUS=4096,
with and without this patch, and compared clang's -fstack-usage output.

With the patch, 62 functions grow by 512 bytes or more. The worst are in
cpuset: update_prstate(), cpuset_write_resmask() and
cpuset_handle_hotplug() grow by ~1.5K, to 1648 bytes. Nothing crosses
FRAME_WARN, and the largest frame in the build goes from 1312 to 1648
bytes, out of a 64K stack.

I believe this higher frame usage is still way better than the kmalloc
at function invocation (the reason for this patch), but honestly I don't
have real numbers, and I am happy to take them, if it is worth.

> Because copying these things by value could be expensive?

Hmm, I _think_ having the cpumask on the stack or not doesn't change how
it is passed. cpumask_var_t is either a pointer or a one-element array,
which decays to a pointer, so callees get a struct cpumask * in both
cases.

Which copies do you have in mind?

Thanks,
--breno

--
PS: I am not pushing on this patch at all, I am just trying to
understand the reasoning and learn.