Re: [PATCH 4/4] lib/cpumask: add FORCE_NR_CPUS config option

[Andy Shevchenko]

On Mon, Aug 29, 2022 at 09:57:48AM -0700, Yury Norov wrote:
> The size of cpumasks is hard-limited by compile-time parameter NR_CPUS,
> but defined at boot-time when kernel parses ACPI/DT tables, and stored in
> nr_cpu_ids. In many practical cases, number of CPUs for a target is known
> at compile time, and can be provided with NR_CPUS.
> 
> In that case, compiler may be instructed to rely on NR_CPUS as on actual
> number of CPUs, not an upper limit. It allows to optimize many cpumask
> routines and significantly shrink size of the kernel image.
> 
> This patch adds FORCE_NR_CPUS option to teach the compiler to rely on
> NR_CPUS and enable corresponding optimizations.
> 
> If FORCE_NR_CPUS=y, kernel will not set nr_cpu_ids at boot, but only check
> that the actual number of possible CPUs is equal to NR_CPUS, and WARN if
> that doesn't hold.
> 
> The new option is especially useful in embedded applications because
> kernel configurations are unique for each SoC, the number of CPUs is
> constant and known well, and memory limitations are typically harder.
> 
> For my 4-CPU ARM64 build with NR_CPUS=4, FORCE_NR_CPUS=y saves 46KB:
>   add/remove: 3/4 grow/shrink: 46/729 up/down: 652/-46952 (-46300)

...

> +       default n

Redundant. 'n' _is_ default.

-- 
With Best Regards,
Andy Shevchenko