Re: [PATCH v2 clocksource 6/7] clocksource: Verify HPET and PMTMR when TSC unverified

[Thomas Gleixner]

On Wed, Feb 01 2023 at 22:40, Waiman Long wrote:
> On 2/1/23 14:55, Paul E. McKenney wrote:
>>>>> Therefore, when watchdog verification of TSC is disabled, enable it for
>>>>> HPET and PMTMR (AKA ACPI PM timer).  This provides the needed in-kernel
>>>>> time-skew diagnostic without degrading the system's performance.
>>>> I'm more than unhappy about this. We finally have a point where the TSC
>>>> watchdog overhead can go away without adding TSC=reliable to the kernel
>>>> commandline.
>>>>
>>>> Now you add an unconditionally enforce the watchdog again in a way which
>>>> even cannot be disabled on the kernel command line.
>>>>
>>>> Patently bad idea, no cookies for you!
>>> I have a similar concern about this patch as well. That is why I was
>>> suggesting to have this enabled for a limited time after boot for sanity
>>> checking purpose only.
>> Fair enough!
>>
>> If the watchdog checking of HPET and/or PMTMR against TSC only happens
>> only when the sysadm asks for it, would you still want to have the ability
>> to enable such watchdog checking at boot time, and then to disable it
>> once the system had been running for some limited time?
>
> Yes, being optional is another way to avoid the overhead for the 
> majority of users. The paranoids can turn it on if they want to.

Yes, opt-in is good enough.

Thanks,

        tglx