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

[Thomas Gleixner <tglx@kernel.org>]

On Thu, Oct 22 2026 at 07:43, Karl Mehltretter wrote:
> On Thu, Oct 15, 2026 at 08:32:25PM +0100, Thomas Gleixner wrote:
>> 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.

Care to send a formal patch? After all you decoded the problem and
suggested the fix. So you should get the credit for it not me.

Thanks,

        tglx