Re: [PATCH clocksource 2/2] clocksource: Exponential backoff for load-induced bogus watchdog reads

[Waiman Long]

On 11/3/22 22:23, Paul E. McKenney wrote:
> On Thu, Nov 03, 2022 at 09:01:45PM -0400, Waiman Long wrote:
> > On 11/3/22 20:26, Paul E. McKenney wrote:
> > > On Thu, Nov 03, 2022 at 08:20:27PM -0400, Waiman Long wrote:
> > > > On 11/3/22 16:49, Paul E. McKenney wrote:
> > > > > commit da44b8af99222ff8761a98ca8c00837a7d607d28
> > > > > Author: Paul E. McKenney<paulmck@xxxxxxxxxx>
> > > > > Date:   Fri Oct 28 10:38:58 2022 -0700
> > > > >
> > > > >        clocksource: Exponential backoff for load-induced bogus watchdog reads
> > > > >        The clocksource watchdog will reject measurements that are excessively
> > > > >        delayed, that is, by more than 1.5 seconds beyond the intended 0.5-second
> > > > >        watchdog interval.  On an extremely busy system, this can result in a
> > > > >        console message being printed every two seconds.  This is excessively
> > > > >        noisy for a non-error condition.
> > > > >        Therefore, apply exponential backoff to these messages.  This exponential
> > > > >        backoff is capped at 1024 times the watchdog interval, which comes to
> > > > >        not quite one message per ten minutes.
> > > > >        Please note that the bogus watchdog reads that occur when the watchdog
> > > > >        interval is less than 0.125 seconds are still printed unconditionally
> > > > >        because these likely correspond to a serious error condition in the
> > > > >        timer code or hardware.
> > > > >        [ paulmck: Apply Feng Tang feedback. ]
> > > > >        [ paulmck: Apply Waiman Long feedback. ]
> > > > >        Reported-by: Waiman Long<longman@xxxxxxxxxx>
> > > > >        Reported-by: Feng Tang<feng.tang@xxxxxxxxx>
> > > > >        Signed-off-by: Paul E. McKenney<paulmck@xxxxxxxxxx>
> > > > >        Cc: John Stultz<jstultz@xxxxxxxxxx>
> > > > >        Cc: Thomas Gleixner<tglx@xxxxxxxxxxxxx>
> > > > >        Cc: Stephen Boyd<sboyd@xxxxxxxxxx>
> > > > >        Cc: Feng Tang<feng.tang@xxxxxxxxx>
> > > > >        Cc: Waiman Long<longman@xxxxxxxxxx>
> > > > >
> > > > > diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
> > > > > index 1d42d4b173271..23b73f2293d6d 100644
> > > > > --- a/include/linux/clocksource.h
> > > > > +++ b/include/linux/clocksource.h
> > > > > @@ -125,6 +125,9 @@ struct clocksource {
> > > > >     	struct list_head	wd_list;
> > > > >     	u64			cs_last;
> > > > >     	u64			wd_last;
> > > > > +	u64			wd_last_bogus;
> > > > > +	int			wd_bogus_shift;
> > > > > +	unsigned long		wd_bogus_count;
> > > > >     #endif
> > > > >     	struct module		*owner;
> > > > >     };
> > > > > diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> > > > > index 3f5317faf891f..1eefb56505350 100644
> > > > > --- a/kernel/time/clocksource.c
> > > > > +++ b/kernel/time/clocksource.c
> > > > > @@ -442,14 +442,33 @@ static void clocksource_watchdog(struct timer_list *unused)
> > > > >     		/* Check for bogus measurements. */
> > > > >     		wdi = jiffies_to_nsecs(WATCHDOG_INTERVAL);
> > > > > -		if (wd_nsec < (wdi >> 2)) {
> > > > > -			/* This usually indicates broken timer code or hardware. */
> > > > > -			pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced only %lld ns during %d-jiffy time interval, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL);
> > > > > +		if (wd_nsec > (wdi << 2) || cs_nsec > (wdi << 2)) {
> > > > > +			bool needwarn = false;
> > > > > +			u64 wd_lb;
> > > > > +
> > > > > +			cs->wd_bogus_count++;
> > > > > +			if (!cs->wd_bogus_shift) {
> > > > > +				needwarn = true;
> > > > > +			} else {
> > > > > +				delta = clocksource_delta(wdnow, cs->wd_last_bogus, watchdog->mask);
> > > > > +				wd_lb = clocksource_cyc2ns(delta, watchdog->mult, watchdog->shift);
> > > > > +				if ((1 << cs->wd_bogus_shift) * wdi <= wd_lb)
> > > > > +					needwarn = true;
> > > > > +			}
> > > > > +			if (needwarn) {
> > > > > +				/* This can happen on busy systems, which can delay the watchdog. */
> > > > > +				pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced an excessive %lld ns during %d-jiffy time interval (%lu additional), probable CPU overutilization, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL, cs->wd_bogus_count);
> > > > Just one question, does "%lu additional" means the number of bogus count
> > > > that doesn't meet the needwarn requirement and hence skipped. If so, I think
> > > > you have to use "cs->wd_bogus_cnt - 1". Other than that, the change looks
> > > > good to me.
> > > It means the number since the last report, or, for the first report,
> > > the number since boot.
> > >
> > > Does that work for you?
> > OK, I think the word "additional" tricks me into thinking about extra bogus
> > messages in additional to the current one. Using another word like "total"
> > may be less confusing.
> My concern with "total" is that people might think that the numbers
> meant the total number of instances since boot.
>
> So how about "(9 since last message)" or similar?
>
> 							Thanx, Paul

Yes, that looks good to me.

Thanks!
Longman