linux-kernel - Re: [PATCH 4/4] lib/cpumask: add FORCE_NR

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Date:   Mon, 29 Aug 2022 20:02:24 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Sander Vanheule <sander@...nheule.net>,
        Alexey Klimov <klimov.linux@...il.com>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH 4/4] lib/cpumask: add FORCE_NR_CPUS config option

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