Re: [PATCH] Clocksource: Avoid misjudgment of clocksource

[Luming Yu <luming.yu@gmail.com>]

On Tue, Oct 19, 2021 at 1:04 AM John Stultz <john.stultz@linaro.org> wrote:
>
> On Mon, Oct 18, 2021 at 9:14 PM yanghui <yanghui.def@bytedance.com> wrote:
> > 在 2021/10/19 上午12:14, John Stultz 写道:
> > > On Tue, Oct 12, 2021 at 1:06 AM brookxu <brookxu.cn@gmail.com> wrote:
> > >> John Stultz wrote on 2021/10/12 13:29:
> > >>> On Mon, Oct 11, 2021 at 10:23 PM brookxu <brookxu.cn@gmail.com> wrote:
> > >>>> John Stultz wrote on 2021/10/12 12:52 下午:
> > >>>>> On Sat, Oct 9, 2021 at 7:04 AM brookxu <brookxu.cn@gmail.com> wrote:
> > >>>> If we record the watchdog's start_time in clocksource_start_watchdog(), and then
> > >>>> when we verify cycles in clocksource_watchdog(), check whether the clocksource
> > >>>> watchdog is blocked. Due to MSB verification, if the blocked time is greater than
> > >>>> half of the watchdog timer max_cycles, then we can safely ignore the current
> > >>>> verification? Do you think this idea is okay?
> > >>>
> > >>> I can't say I totally understand the idea. Maybe could you clarify with a patch?
> > >>>
> > >>
> > >> Sorry, it looks almost as follows:
> > >>
> > >> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> > >> index b8a14d2..87f3b67 100644
> > >> --- a/kernel/time/clocksource.c
> > >> +++ b/kernel/time/clocksource.c
> > >> @@ -119,6 +119,7 @@
> > >>   static DECLARE_WORK(watchdog_work, clocksource_watchdog_work);
> > >>   static DEFINE_SPINLOCK(watchdog_lock);
> > >>   static int watchdog_running;
> > >> +static unsigned long watchdog_start_time;
> > >>   static atomic_t watchdog_reset_pending;
> > >>
> > >>   static inline void clocksource_watchdog_lock(unsigned long *flags)
> > >> @@ -356,6 +357,7 @@ static void clocksource_watchdog(struct timer_list *unused)
> > >>          int next_cpu, reset_pending;
> > >>          int64_t wd_nsec, cs_nsec;
> > >>          struct clocksource *cs;
> > >> +       unsigned long max_jiffies;
> > >>          u32 md;
> > >>
> > >>          spin_lock(&watchdog_lock);
> > >> @@ -402,6 +404,10 @@ static void clocksource_watchdog(struct timer_list *unused)
> > >>                  if (atomic_read(&watchdog_reset_pending))
> > >>                          continue;
> > >>
> > >> +               max_jiffies = nsecs_to_jiffies(cs->max_idle_ns);
> > >> +               if (time_is_before_jiffies(watchdog_start_time + max_jiffies))
> > >> +                       continue;
> > >> +
> > >
> > > Sorry, what is the benefit of using jiffies here?   Jiffies are
> > > updated by counting the number of tick intervals on the current
> > > clocksource.
> > >
> > > This seems like circular logic, where we're trying to judge the
> > > current clocksource by using something we derived from the current
> > > clocksource.
> > > That's why the watchdog clocksource is important, as it's supposed to
> > > be a separate counter that is more reliable (but likely slower) then
> > > the preferred clocksource.
> > >
> > > So I'm not really sure how this helps.
> > >
> > > The earlier patch by yanghui at least used the watchdog interval to
> > > decide if the watchdog timer had expired late. Which seemed
> > > reasonable, but I thought it might be helpful to add some sort of a
> > > counter so if the case is happening repeatedly (timers constantly
> > > being delayed) we have a better signal that the watchdog and current
> > > clocksource are out of sync.  Because again, timers are fired based on
> >
> > I think only have a signal ls not enough. we need to prevent
> > clocksource from being incorrectly switched.
>
> Right, but we also have to ensure that we also properly disqualify
> clocksources that are misbehaving.
>
> In the case that the current clocksource is running very slow (imagine
> old TSCs that lowered freq with cpufreq), then system time slows down,
> so timers fire late.
> So it would constantly seem like the irqs are being delayed, so with
> your logic we would not disqualify a clearly malfunctioning
> clocksource..
>
> > The Timer callback function clocksource_watchdog() is executed in the
> > context of softirq(run_timer_softirq()). So if softirq is disabled for
> > long time(One situation is long time softlockup), clocksource_watchdog()
> > will be delay executed.
>
> Yes. The reality is that timers are often spuriously delayed. We don't
> want a short burst of timer misbehavior to disqualify a good
> clocksource.
>
> But the problem is that this situation and the one above (with the
> freq changing TSC), will look exactly the same.
>
> So having a situation where if the watchdog clocksource thinks too
> much time has passed between watchdog timers, we can skip judgement,
> assuming its a spurious delay. But I think we need to keep a counter
> so that if this happens 3-5 times in a row, we stop ignoring the
> misbehavior and judge the current clocksource, as it may be running
> slowly.
>
> > >
> > I think it will be better to add this to my patch:
> >   /*
> >    * Interval: 0.5sec.
> > - * MaxInterval: 1s.
> > + * MaxInterval: 20s.
> >    */
> >   #define WATCHDOG_INTERVAL (HZ >> 1)
> > -#define WATCHDOG_MAX_INTERVAL_NS (NSEC_PER_SEC)
> > +#define WATCHDOG_MAX_INTERVAL_NS (20 * NSEC_PER_SEC)
> >
>
> Some watchdog counters wrap within 20 seconds, so I don't think this
> is a good idea.
>
> The other proposal to calculate the error rate, rather than a fixed
> error boundary might be useful too, as if the current clocksource and
> watchdog are close, a long timer delay won't disqualify them if we
> scale the error bounds to be within an given error rate.

In most of tsc unstable trouble shooting on modern servers we experienced,
it usually ends up in a false alarm triggered by the  clock source
watchdog for tsc.

I think Paul has a proposal to make a clock source watchdog to be more
intelligent.
Its job is to find a real problem instead of causing a problem.

so disabling it for known good-tsc might be a reasonable good idea
that can save manpower for other
more valuable problems to solve, or at least make it statistically a
problem less chance to happen.
>
> thanks
> -john