Re: [REGRESSION] clocksource: delayed watchdog can mark TSC unstable

[Karl Mehltretter]

On Thu, Oct 15, 2026 at 08:32:25PM +0100, Thomas Gleixner wrote:
> >   busybox-79 [000] ..s1. 15.168486: clocksource_watchdog <-call_timer_fn
> >   busybox-79 [000] .Ns.. 16.269726: clocksource_watchdog <-call_timer_fn
> >   busybox-79 [000] .Ns.. 16.269820: clocksource_watchdog <-call_timer_fn
> 
> I did not think about that scenario obvioulsy. The code is resilient
> against long intervals between two measurements, so the easy and
> straight forward fix is indeed what you (or the LLM) suggested.
> 
> Thanks,
> 
>         tglx
> ---
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -673,7 +673,7 @@ static void clocksource_watchdog(struct
>  
>  	/* Could have been rearmed by a stop/start cycle */
>  	if (!timer_pending(&watchdog_timer)) {
> -		watchdog_timer.expires += WATCHDOG_INTERVAL;
> +		watchdog_timer.expires = jiffies + WATCHDOG_INTERVAL;
>  		add_timer_local(&watchdog_timer);
>  	}
>  }

Yes, I tested this change on v7.3-rc5 (72d3fcf802c4)

                              baseline   patched
TSC marked unstable           2/3 boots  0/3 boots
watchdog gaps below 250 ms    3/3 boots  0/3 boots
minimum traced gap            0.231 ms   505.484 ms

Each boot had four QEMU TCG vCPUs. I used PTRACE_ATTACH to
stop only the vCPU 0 TCG thread for 800 ms.

Thanks,
Karl