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

[Waiman Long <longman@redhat.com>]

On 11/4/22 22:38, Paul E. McKenney wrote:
> On Fri, Nov 04, 2022 at 09:55:02AM -0400, Waiman Long wrote:
>> 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:
> [ . . . ]
>
>>>>>>> +					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.
> Thank you, and please see below for the updated patch.
>
> 							Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit a7dc308e8359eafb58df360e06b66ecbf79a4d0b
> 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.
>      
>      [ paulmck: Apply Feng Tang feedback. ]
>      [ paulmck: Apply Waiman Long feedback. ]
>      
>      Reported-by: Waiman Long <longman@redhat.com>
>      Reported-by: Feng Tang <feng.tang@intel.com>
>      Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>      Reviewed-by: Feng Tang <feng.tang@intel.com>
>      Cc: John Stultz <jstultz@google.com>
>      Cc: Thomas Gleixner <tglx@linutronix.de>
>      Cc: Stephen Boyd <sboyd@kernel.org>
>      Cc: Waiman Long <longman@redhat.com>
>
> 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..4015ec6503a52 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 since last message), probable CPU overutilization, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL, cs->wd_bogus_count);
> +				cs->wd_last_bogus = wdnow;
> +				if (cs->wd_bogus_shift < 10)
> +					cs->wd_bogus_shift++;
> +				cs->wd_bogus_count = 0;
> +			}
>   			continue;
>   		}
> -		if (wd_nsec > (wdi << 2)) {
> -			/* 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);
> +		/* Check too-short measurements second to handle wrap. */
> +		if (wd_nsec < (wdi >> 2) || 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);
>   			continue;
>   		}
>   
>
Looks good to me.

Reviewed-by: Waiman Long <longman@redhat.com>