Re: [PATCH v5 13/18] watchdog/hardlockup: Have the perf hardlockup use __weak functions more cleanly

[Doug Anderson]

Hi,

On Wed, May 24, 2023 at 6:59 AM Petr Mladek <pmladek@xxxxxxxx> wrote:
>
> On Fri 2023-05-19 10:18:37, Douglas Anderson wrote:
> > The fact that there watchdog_hardlockup_enable(),
> > watchdog_hardlockup_disable(), and watchdog_hardlockup_probe() are
> > declared __weak means that the configured hardlockup detector can
> > define non-weak versions of those functions if it needs to. Instead of
> > doing this, the perf hardlockup detector hooked itself into the
> > default __weak implementation, which was a bit awkward. Clean this up.
> >
> > >From comments, it looks as if the original design was done because the
> > __weak function were expected to implemented by the architecture and
> > not by the configured hardlockup detector. This got awkward when we
> > tried to add the buddy lockup detector which was not arch-specific but
> > wanted to hook into those same functions.
> >
> > This is not expected to have any functional impact.
> >
> > @@ -187,27 +187,33 @@ static inline void watchdog_hardlockup_kick(void) { }
> >  #endif /* !CONFIG_HARDLOCKUP_DETECTOR_PERF */
> >
> >  /*
> > - * These functions can be overridden if an architecture implements its
> > - * own hardlockup detector.
> > + * These functions can be overridden based on the configured hardlockdup detector.
> >   *
> >   * watchdog_hardlockup_enable/disable can be implemented to start and stop when
> > - * softlockup watchdog start and stop. The arch must select the
> > + * softlockup watchdog start and stop. The detector must select the
> >   * SOFTLOCKUP_DETECTOR Kconfig.
> >   */
> > -void __weak watchdog_hardlockup_enable(unsigned int cpu)
> > -{
> > -     hardlockup_detector_perf_enable();
> > -}
> > +void __weak watchdog_hardlockup_enable(unsigned int cpu) { }
> >
> > -void __weak watchdog_hardlockup_disable(unsigned int cpu)
> > -{
> > -     hardlockup_detector_perf_disable();
> > -}
> > +void __weak watchdog_hardlockup_disable(unsigned int cpu) { }
> >
> >  /* Return 0, if a hardlockup watchdog is available. Error code otherwise */
> >  int __weak __init watchdog_hardlockup_probe(void)
> >  {
> > -     return hardlockup_detector_perf_init();
> > +     /*
> > +      * If CONFIG_HAVE_NMI_WATCHDOG is defined then an architecture
> > +      * is assumed to have the hard watchdog available and we return 0.
> > +      */
> > +     if (IS_ENABLED(CONFIG_HAVE_NMI_WATCHDOG))
> > +             return 0;
> > +
> > +     /*
> > +      * Hardlockup detectors other than those using CONFIG_HAVE_NMI_WATCHDOG
> > +      * are required to implement a non-weak version of this probe function
> > +      * to tell whether they are available. If they don't override then
> > +      * we'll return -ENODEV.
> > +      */
> > +     return -ENODEV;
> >  }
>
> When thinking more about it. It is weird that we need to handle
> CONFIG_HAVE_NMI_WATCHDOG in this default week function.
>
> It should be handled in watchdog_hardlockup_probe() implemented
> in kernel/watchdog_perf.c.
>
> IMHO, the default __weak function could always return -ENODEV;
>
> Would it make sense, please?

I don't quite understand. I'd agree that the special case for
CONFIG_HAVE_NMI_WATCHDOG is ugly, but it was also ugly before. IMO
it's actually a little less ugly / easier to understand after my
patch. ...but let me walk through how I think this special case works
and maybe you can tell me where I'm confused.

The first thing to understand is that CONFIG_HARDLOCKUP_DETECTOR_PERF
and CONFIG_HAVE_NMI_WATCHDOG are mutually exclusive from each other.
This was true before any of my patches and is still true after them.
Specifically, if CONFIG_HAVE_NMI_WATCHDOG is defined then an
architecture implements arch_touch_nmi_watchdog() (as documented in
the Kconfig docs for HAVE_NMI_WATCHDOG). Looking at the tree before my
series you can see that the perf hardlockup detector also implemented
arch_touch_nmi_watchdog(). This would have caused a conflict. The
mutual exclusion was presumably enforced by an architecture not
defining both HAVE_NMI_WATCHDOG and HAVE_HARDLOCKUP_DETECTOR_PERF.

The second thing to understand is that an architecture that defines
CONFIG_HAVE_NMI_WATCHDOG is _not_ required to implement
watchdog_hardlockup_probe() (used to be called watchdog_nmi_probe()).
Maybe this should change, but at the very least it appears that
SPARC64 defines HAVE_NMI_WATCHDOG but doesn't define
watchdog_hardlockup_probe() AKA watchdog_nmi_probe(). Anyone who
defines CONFIG_HAVE_NMI_WATCHDOG and doesn't implement
watchdog_hardlockup_probe() is claiming that their watchdog needs no
probing and is always available.

So with that context:

1. We can't handle any special cases for CONFIG_HAVE_NMI_WATCHDOG in
"kernel/watchdog_perf.c". The special cases that we need to handle are
all for the cases where CONFIG_HARDLOCKUP_DETECTOR_PERF isn't defined
and that means "kernel/watchdog_perf.c" isn't included.

2. We can't have the default __weak function return -ENODEV because
CONFIG_HAVE_NMI_WATCHDOG doesn't require an arch to implement
watchdog_hardlockup_probe(), but we want watchdog_hardlockup_probe()
to return "no error" in that case so that
"watchdog_hardlockup_available" gets set to true.

Does that sound right?

I'd agree that a future improvement saying that
CONFIG_HAVE_NMI_WATCHDOG means you _must_ implement
watchdog_hardlockup_probe() would make sense and that would allow us
to get rid of the special case. IMO, though, that's a separate patch.
I'd be happy to review that patch if you wanted to post it up. :-)

If we want to add that requirement, I _think_ the only thing you'd
need to do is to add watchdog_hardlockup_probe() to sparc64 and have
it return 0 and put that definition in the same file containing
arch_touch_nmi_watchdog(). powerpc also gets CONFIG_HAVE_NMI_WATCHDOG
as a side effect of selecting CONFIG_HAVE_HARDLOCKUP_DETECTOR_ARCH but
it looks like they implement watchdog_hardlockup_probe() already. Oh,
but maybe this will fix a preexisting (existed before my patches)
minor bug... Unless I'm missing something (entirely possible!) on
powerpc today I guess if you turn off CONFIG_PPC_WATCHDOG then
CONFIG_HAVE_HARDLOCKUP_DETECTOR_ARCH and CONFIG_HAVE_NMI_WATCHDOG
would still be defined and we'd end up returning 0 (no error) from
watchdog_hardlockup_probe(). That means that on powerpc today if you
turn off CONFIG_PPC_WATCHDOG that '/proc/sys/kernel/nmi_watchdog' will
still think the watchdog is enabled?


-Doug