Re: [PATCH clocksource] Reject bogus watchdog clocksource measurements

[Feng Tang <feng.tang@intel.com>]

On Fri, Oct 28, 2022 at 10:52:45AM -0700, Paul E. McKenney wrote:
[...]
> > > 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!
> 
> And here is the resulting patch.  Thoughts?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit adc0512cf34fe7f0d73966d59644b826ee526742
> Author: Paul E. McKenney <paulmck@kernel.org>
> 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.
>     
>     Reported-by: Waiman Long <longman@redhat.com>
>     Reported-by: Feng Tang <feng.tang@intel.com>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>     Cc: John Stultz <jstultz@google.com>
>     Cc: Thomas Gleixner <tglx@linutronix.de>
>     Cc: Stephen Boyd <sboyd@kernel.org>
>     Cc: Feng Tang <feng.tang@intel.com>
>     Cc: Waiman Long <longman@redhat.com>
> 
> diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
> index 1d42d4b173271..daac05aedf56a 100644
> --- a/include/linux/clocksource.h
> +++ b/include/linux/clocksource.h
> @@ -125,6 +125,10 @@ 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;
> +	unsigned long		wd_bogus_count_last;
>  #endif
>  	struct module		*owner;
>  };
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index 3f5317faf891f..6537ffa02e445 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -448,8 +448,26 @@ static void clocksource_watchdog(struct timer_list *unused)
>  			continue;
>  		}
>  		if (wd_nsec > (wdi << 2)) {

Just recalled one thing, that it may be better to check 'cs_nsec' 
instead of 'wd_nsec', as some watchdog may have small wrap-around
value. IIRC, HPET's counter is 32 bits long and wraps at about
300 seconds, and PMTIMER's counter is 24 bits which wraps at about
3 ~ 4 seconds. So when a long stall of the watchdog timer happens,
the watchdog's value could 'overflow' many times.

And usually the 'current' closcksource has longer wrap time than
the watchdog.


> -			/* 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, probable CPU overutilization, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL);
> +			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;

I'm not sure if we need to check the last_bogus counter, or just
the current interval 'cs_nsec' is what we care, and some code
like this ?

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index daac05aedf56..3910dbb9b960 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -125,7 +125,6 @@ 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;
 	unsigned long		wd_bogus_count_last;
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 6537ffa02e44..8e6d498b1492 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -442,28 +442,18 @@ static void clocksource_watchdog(struct timer_list *unused)
 
 		/* Check for bogus measurements. */
 		wdi = jiffies_to_nsecs(WATCHDOG_INTERVAL);
-		if (wd_nsec < (wdi >> 2)) {
+		if (cs_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);
+			pr_warn("timekeeping watchdog on CPU%d: clocksource '%s' advanced only %lld ns during %d-jiffy time interval, skipping watchdog check.\n", smp_processor_id(), cs->name, wd_nsec, WATCHDOG_INTERVAL);
 			continue;
 		}
-		if (wd_nsec > (wdi << 2)) {
-			bool needwarn = false;
-			u64 wd_lb;
-
+		if (cs_nsec > (wdi << 2)) {
 			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) {
+			if (!cs->wd_bogus_shift ||
+			    (1 << cs->wd_bogus_shift) * wdi <= cs_nsec) {
 				/* 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 - cs->wd_bogus_count_last);
-				cs->wd_last_bogus = wdnow;
+				pr_warn("timekeeping watchdog on CPU%d: clocksource '%s' advanced an excessive %lld ns during %d-jiffy time interval (%lu additional), probable CPU overutilization, skipping watchdog check.\n", smp_processor_id(), cs->name, cs_nsec, WATCHDOG_INTERVAL, cs->wd_bogus_count - cs->wd_bogus_count_last);
+
 				if (cs->wd_bogus_shift < 10)
 					cs->wd_bogus_shift++;
 				cs->wd_bogus_count_last = cs->wd_bogus_count;

Thanks,
Feng


> +			}
> +			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 - cs->wd_bogus_count_last);
> +				cs->wd_last_bogus = wdnow;
> +				if (cs->wd_bogus_shift < 10)
> +					cs->wd_bogus_shift++;
> +				cs->wd_bogus_count_last = cs->wd_bogus_count;
> +			}
>  			continue;
>  		}
>