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

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Message-ID: <CAHk-=wjSVUpjh0UZW+w0C633gQo=-JhGH9ja8kUmRW0TUcSMpw@mail.gmail.com>
Date:   Mon, 7 Nov 2022 09:55:34 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.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>
Subject: Re: [PATCH v2] cpumask: limit visibility of FORCE_NR_CPUS

On Mon, Nov 7, 2022 at 4:45 AM Valentin Schneider <vschneid@...hat.com> wrote:
>
> True, this would have been neater as a single config, but AIUI it's a
> required "trick" for allyesconfig. I would have expected other configs to
> have hit similar issues in the past, but didn't find any.

Actually, the standard trick for allmodconfig and allyesconfig is to
use the "COMPILE_TEST" config variable.

It's basically a variable for "I'm not going to *run* the result, but
I want to make sure to get build coverage".

And both allmodconfig and allyesconfig set that config option.

In most cases, the "COMPILE_TEST" config variable is used to enable
things that wouldn't make sense on the chosen hardware platform, so
you have things like

        depends on ARCH_DAVINCI || COMPILE_TEST

because some driver only makes sense on ARCH_DAVINCI, but people still
want the build coverage.

But sometimes it's used the other way around, so fro example on x86 we have

     config X86_DECODER_SELFTEST

which explicitly depends on COMPILE_TEST *not* being set, because it's
a test that takes forever to run (particularly for huge kernels), and
so it's actually disabled for the  common all{yes,mod}config cases.

Same goes for things like LTO_CLANG_FULL. It's just expensive for big
build tests, plus causes too many issues for now.

End result: if some option actually *reduces* test coverage, or has
some other reason why it makes no sense for build tests, use that

        depends on !COMPILE_TEST

to not have allmodconfig and allyesconfig pick it.

                    Linus