Re: [PATCH v4 0/2] x86/tsc: Exempt recent UV systems from clocksource watchdog checks to avoid false positives.

[Dimitri Sivanich]

On Thu, May 21, 2026 at 09:30:14PM +0200, Thomas Gleixner wrote:
> On Thu, May 21 2026 at 08:17, Dimitri Sivanich wrote:
> > HPE UV hardware and firmware is designed to ensure a reliable and
> > synchronized TSC mechanism.  Comparing the TSC against secondary
> > clocksources can result in false positives due to variable access
> > latency caused by system traffic.  The best course of action against
> > these false positives has been found to simply disable watchdog
> > checking of the TSC.
> >
> > Commits [1] and [2] were introduced to avoid an issue where the TSC
> > is falsely declared unstable by exempting qualified platforms of up
> > to 4-sockets from TSC clocksource watchdog checking.  Extend that
> > exemption to include recent and future UV platforms.
> 
> Jiri asked you in the V3 submission:
> 
>  "A new implementation of the clocksource watchdog has been merged into
>   the upstream kernel. One of the changes made by the new clocksource
>   watchdog implementation is that reference clocksource reads are made
>   on the boot CPU only. Perhaps, the sgi_rtc clocksource would work well
>   with this implementation. So, testing is needed in order to find out
>   if this patch are any future in the upstream Linux. Dimitri, would you
>   be able to run tests on UV systems to check if the new clocksource
>   watchdog implementation works and the hardware limitations of sgi_rtc
>   do not get in the way?"
> 
> This question is still not answered by you and it has been confirmed
> that the new watchdog works flawlessly on a 1920 threads 16 socket
> system under massive load and system traffic.

I tested a 7.1-rc4 kernel on a 2048 thread 16 socket system and, while
under test, the TSC did get marked as unstable after a series of "sgi_rtc
read timed out" warnings.

> 
> So you do not even have the courtesy to test, you just go and make the
> same claims you made before based on the original watchdog
> implementation.
> 
> Feel free to ignore people, but then don't be surprised when people
> ignore you as well.
> 
> Thanks,
> 
>         tglx