Re: [PATCH v5 10/18] watchdog/hardlockup: Add a "cpu" param to watchdog_hardlockup_check()

[Doug Anderson]

Hi,

On Tue, May 23, 2023 at 9:02 AM Petr Mladek <pmladek@xxxxxxxx> wrote:
>
> On Fri 2023-05-19 10:18:34, Douglas Anderson wrote:
> > In preparation for the buddy hardlockup detector where the CPU
> > checking for lockup might not be the currently running CPU, add a
> > "cpu" parameter to watchdog_hardlockup_check().
> >
> > As part of this change, make hrtimer_interrupts an atomic_t since now
> > the CPU incrementing the value and the CPU reading the value might be
> > different. Technially this could also be done with just READ_ONCE and
> > WRITE_ONCE, but atomic_t feels a little cleaner in this case.
> >
> > While hrtimer_interrupts is made atomic_t, we change
> > hrtimer_interrupts_saved from "unsigned long" to "int". The "int" is
> > needed to match the data type backing atomic_t for hrtimer_interrupts.
> > Even if this changes us from 64-bits to 32-bits (which I don't think
> > is true for most compilers), it doesn't really matter. All we ever do
> > is increment it every few seconds and compare it to an old value so
> > 32-bits is fine (even 16-bits would be). The "signed" vs "unsigned"
> > also doesn't matter for simple equality comparisons.
> >
> > hrtimer_interrupts_saved is _not_ switched to atomic_t nor even
> > accessed with READ_ONCE / WRITE_ONCE. The hrtimer_interrupts_saved is
> > always consistently accessed with the same CPU. NOTE: with the
> > upcoming "buddy" detector there is one special case. When a CPU goes
> > offline/online then we can change which CPU is the one to consistently
> > access a given instance of hrtimer_interrupts_saved. We still can't
> > end up with a partially updated hrtimer_interrupts_saved, however,
> > because we end up petting all affected CPUs to make sure the new and
> > old CPU can't end up somehow read/write hrtimer_interrupts_saved at
> > the same time.
> >
> > --- a/kernel/watchdog.c
> > +++ b/kernel/watchdog.c
> > @@ -87,29 +87,34 @@ __setup("nmi_watchdog=", hardlockup_panic_setup);
> >
> >  #if defined(CONFIG_HARDLOCKUP_DETECTOR_PERF)
> >
> > -static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts);
> > -static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts_saved);
> > +static DEFINE_PER_CPU(atomic_t, hrtimer_interrupts);
> > +static DEFINE_PER_CPU(int, hrtimer_interrupts_saved);
> >  static DEFINE_PER_CPU(bool, watchdog_hardlockup_warned);
> >  static unsigned long watchdog_hardlockup_all_cpu_dumped;
> >
> > -static bool is_hardlockup(void)
> > +static bool is_hardlockup(unsigned int cpu)
> >  {
> > -     unsigned long hrint = __this_cpu_read(hrtimer_interrupts);
> > +     int hrint = atomic_read(&per_cpu(hrtimer_interrupts, cpu));
> >
> > -     if (__this_cpu_read(hrtimer_interrupts_saved) == hrint)
> > +     if (per_cpu(hrtimer_interrupts_saved, cpu) == hrint)
> >               return true;
> >
> > -     __this_cpu_write(hrtimer_interrupts_saved, hrint);
> > +     /*
> > +      * NOTE: we don't need any fancy atomic_t or READ_ONCE/WRITE_ONCE
> > +      * for hrtimer_interrupts_saved. hrtimer_interrupts_saved is
> > +      * written/read by a single CPU.
> > +      */
> > +     per_cpu(hrtimer_interrupts_saved, cpu) = hrint;
> >
> >       return false;
> >  }
> >
> >  static void watchdog_hardlockup_kick(void)
> >  {
> > -     __this_cpu_inc(hrtimer_interrupts);
> > +     atomic_inc(raw_cpu_ptr(&hrtimer_interrupts));
>
> Is there any particular reason why raw_*() is needed, please?
>
> My expectation is that the raw_ API should be used only when
> there is a good reason for it. Where a good reason might be
> when the checks might fail but the consistency is guaranteed
> another way.
>
> IMHO, we should use:
>
>         atomic_inc(this_cpu_ptr(&hrtimer_interrupts));
>
> To be honest, I am a bit lost in the per_cpu API definitions.
>
> But this_cpu_ptr() seems to be implemented the same way as
> per_cpu_ptr() when CONFIG_DEBUG_PREEMPT is enabled.
> And we use per_cpu_ptr() in is_hardlockup().
>
> Also this_cpu_ptr() is used more commonly:
>
> $> git grep this_cpu_ptr | wc -l
> 1385
> $> git grep raw_cpu_ptr | wc -l
> 114

Hmmm, I think maybe I confused myself. The old code purposely used the
double-underscore prefixed version of this_cpu_inc(). I couldn't find
a double-underscore version of this_cpu_ptr() and I somehow convinced
myself that the raw() version was the right equivalent version.

You're right that this_cpu_ptr() is a better choice here and I don't
see any reason why we'd need the raw version.

> >  }
> >
> > -void watchdog_hardlockup_check(struct pt_regs *regs)
> > +void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
> >  {
> >       /*
> >        * Check for a hardlockup by making sure the CPU's timer
> > @@ -117,35 +122,42 @@ void watchdog_hardlockup_check(struct pt_regs *regs)
> >        * fired multiple times before we overflow'd. If it hasn't
> >        * then this is a good indication the cpu is stuck
> >        */
> > -     if (is_hardlockup()) {
> > +     if (is_hardlockup(cpu)) {
> >               unsigned int this_cpu = smp_processor_id();
> > +             struct cpumask backtrace_mask = *cpu_online_mask;
>
> Does this work, please?
>
> IMHO, we should use cpumask_copy().

Ah, good call, thanks!


> >               /* Only print hardlockups once. */
> > -             if (__this_cpu_read(watchdog_hardlockup_warned))
> > +             if (per_cpu(watchdog_hardlockup_warned, cpu))
> >                       return;
> >
>
> Otherwise, it looks good to me.

Neither change seems urgent though both are important to fix, I'll
wait a day or two to see if you have feedback on any of the other
patches and I'll send a fixup series.

-Doug