linux-kernel - Re: [PATCH 2/7] watchdog/hardlockup: Make the config checks more straightforward

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Message-ID: <CAD=FV=XzueJia--Zv4cAofzk7yocmP-7K8wa4doAN8pzED_hZA@mail.gmail.com>
Date:   Thu, 8 Jun 2023 06:55:23 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        Nicholas Piggin <npiggin@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        sparclinux@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 2/7] watchdog/hardlockup: Make the config checks more straightforward

Hi,

On Thu, Jun 8, 2023 at 4:02 AM Petr Mladek <pmladek@...e.com> wrote:
>
> > >  config HARDLOCKUP_DETECTOR
> > >         bool "Detect Hard Lockups"
> > >         depends on DEBUG_KERNEL && !S390
> > > -       depends on HAVE_HARDLOCKUP_DETECTOR_NON_ARCH || HAVE_HARDLOCKUP_DETECTOR_ARCH
> > > +       depends on ((HAVE_HARDLOCKUP_DETECTOR_PERF || HAVE_HARDLOCKUP_DETECTOR_BUDDY) && !HAVE_NMI_WATCHDOG) || HAVE_HARDLOCKUP_DETECTOR_ARCH
> >
> > Adding the dependency to buddy (see ablove) would simplify the above
> > to just this:
> >
> > depends on HAVE_HARDLOCKUP_DETECTOR_PERF ||
> > HAVE_HARDLOCKUP_DETECTOR_BUDDY || HAVE_HARDLOCKUP_DETECTOR_ARCH
>
> This is exactly what I do not want. It would just move the check
> somewhere else. But it would make the logic harder to understand.

Hmmm. To me, it felt easier to understand by moving this into the
"HAVE_HARDLOCKUP_DETECTOR_BUDDY". To me it was pretty easy to say "if
an architecture defined its own arch-specific watchdog then buddy
can't be enabled" and that felt like it fit cleanly within the
"HAVE_HARDLOCKUP_DETECTOR_BUDDY" definition. It got rid of _a lot_ of
other special cases / checks elsewhere and felt quite a bit cleaner to
me. I only had to think about the conflict between the "buddy" and
"nmi" watchdogs once when I understood
"HAVE_HARDLOCKUP_DETECTOR_BUDDY".

> > As per above, it's simply a responsibility of architectures not to
> > define that they have both "perf" if they have the NMI watchdog, so
> > it's just buddy to worry about.
>
> Where is this documented, please?
> Is it safe to assume this?

It's not well documented and I agree that it could be improved. Right
now, HAVE_NMI_WATCHDOG is documented to say that the architecture
"defines its own arch_touch_nmi_watchdog()". Looking before my
patches, you can see that "kernel/watchdog_hld.c" (the "perf" detector
code) unconditionally defines arch_touch_nmi_watchdog(). That would
give you a linker error.

> I would personally prefer to ensure this by the config check.
> It is even better than documentation because nobody reads
> documentation ;-)

Sure. IMO this should be documented as close as possible to the root
of the problem. Make "HAVE_NMI_WATCHDOG" depend on
"!HAVE_HARDLOCKUP_DETECTOR_PERF". That expresses that an architecture
is not allowed to declare that it has both.