linux-kernel - Re: [PATCH v2] cpumask: limit visibility of FORCE_NR

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Message-ID: <xhsmhtu3evcme.mognet@vschneid.remote.csb>
Date:   Fri, 04 Nov 2022 17:43:53 +0000
From:   Valentin Schneider <vschneid@...hat.com>
To:     Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        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>,
        Sander Vanheule <sander@...nheule.net>,
        Alexey Klimov <klimov.linux@...il.com>,
        Eric Biggers <ebiggers@...gle.com>
Cc:     Yury Norov <yury.norov@...il.com>
Subject: Re: [PATCH v2] cpumask: limit visibility of FORCE_NR_CPUS

On 19/10/22 15:59, Yury Norov wrote:
> In current form, FORCE_NR_CPUS is visible to all users building their
> kernels, even not experts. It is also set in allmodconfig or allyesconfig,
> which is not a correct behavior.
>
> The 'choice' and unused config UNFORCE_NR_CPUS are used to ensure that
> auto-generated configs that try to enable as much options as possible,
> like allmodconfig, don't enable FORCE_NR_CPUS.
>
> Suggested-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> ---
> v2: extend commit message with an explanation for what we need 'choice'.
>
>  lib/Kconfig | 31 ++++++++++++++++++++++++-------
>  1 file changed, 24 insertions(+), 7 deletions(-)
>
> diff --git a/lib/Kconfig b/lib/Kconfig
> index 9bbf8a4b2108..1ada12f5dda6 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -528,14 +528,31 @@ config CPUMASK_OFFSTACK
>         them on the stack.  This is a bit more expensive, but avoids
>         stack overflow.
>
> +choice
> +	prompt "Number of CPUs detection method"
> +	default UNFORCE_NR_CPUS
> +	depends on SMP && EXPERT

What about moving the 'depends on EXPERT' onto FORCE_NR_CPUS? I find it
makes it easier to figure out the requirements for that option, and is
similar to how e.g. CONFIG_PREEMPT_RT is handled.

> +	help
> +	  Select between boot-time and compile-time detection of number
> +	  of CPUs. If it's possible to provide exact number of CPUs at
> +	  compile-time, kernel code may be optimized better.
> +	  For general-purpose kernel, choose "boot time" option.
> +
> +config UNFORCE_NR_CPUS
> +	bool "Set number of CPUs at boot time"
> +	help
> +	  Choose it if you build general-purpose kernel and want to rely
> +	  on kernel to detect actual number of CPUs.
> +
>  config FORCE_NR_CPUS
> -       bool "NR_CPUS is set to an actual number of CPUs"
> -       depends on SMP
> -       help
> -         Say Yes if you have NR_CPUS set to an actual number of possible
> -         CPUs in your system, not to a default value. This forces the core
> -         code to rely on compile-time value and optimize kernel routines
> -         better.
> +	bool "Set number of CPUs at compile time"
> +	help
> +	  Choose it if NR_CPUS corresponds to an actual number of
> +	  possible CPUs in your system. This forces the core code
> +	  to rely on compile-time value and optimize kernel routines
> +	  better.
> +
> +endchoice
>
>  config CPU_RMAP
>       bool
> --
> 2.34.1