Re: [PATCH clocksource] Reject bogus watchdog clocksource measurements

[Feng Tang]

On Thu, Oct 20, 2022 at 07:09:44AM -0700, Paul E. McKenney wrote:
[...]
> > >  static void clocksource_watchdog(struct timer_list *unused)
> > >  {
> > > -	u64 csnow, wdnow, cslast, wdlast, delta;
> > > +	u64 csnow, wdnow, cslast, wdlast, delta, wdi;
> > >  	int next_cpu, reset_pending;
> > >  	int64_t wd_nsec, cs_nsec;
> > >  	struct clocksource *cs;
> > > @@ -440,6 +440,17 @@ static void clocksource_watchdog(struct timer_list *unused)
> > >  		if (atomic_read(&watchdog_reset_pending))
> > >  			continue;
> > >  
> > > +		/* Check for bogus measurements. */
> > > +		wdi = jiffies_to_nsecs(WATCHDOG_INTERVAL);
> > > +		if (wd_nsec < (wdi >> 2)) {
> > > +			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);
> > > +			continue;
> > > +		}
> > 
> > If this happens (500ms timer happens only after less than 125ms),
> > there is some severe problem with timer/interrupt system. 
> 
> Should I add ", suspect timer/interrupt bug" just after "jiffy time
> interval"?  Or would a comment before that pr_warn() work better for you?

Both are fine for me.

> > > +		if (wd_nsec > (wdi << 2)) {
> > > +			pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced an excessive %lld ns during %d-jiffy time interval, probable CPU overutilization, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL);
> > > +			continue;
> > > +		}
> > 
> > I agree with Waiman that some rate limiting may be needed. As there
> > were reports of hundreds of seconds of delay, 2 seconds delay could
> > easily happen if a system is too busy or misbehave to trigger this
> > problem.
> 
> Good points, thank you both!
> 
> Left to myself, I would use a capped power-of-two backoff that was reset
> any time that the interval was within bounds.  Maybe a cap of 10 minutes?

Yep, that's more informative. Thanks!

- Feng

> Or is there a better way to do this?
> 
> 							Thanx, Paul