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

[Waiman Long]

On 2/1/23 05:24, Thomas Gleixner wrote:
> Paul!
>
> On Tue, Jan 24 2023 at 16:27, Paul E. McKenney wrote:
> > On systems with two or fewer sockets, when the boot CPU has CONSTANT_TSC,
> > NONSTOP_TSC, and TSC_ADJUST, clocksource watchdog verification of the
> > TSC is disabled.  This works well much of the time, but there is the
> > occasional production-level system that meets all of these criteria, but
> > which still has a TSC that skews significantly from atomic-clock time.
> > This is usually attributed to a firmware or hardware fault.  Yes, the
> > various NTP daemons do express their opinions of userspace-to-atomic-clock
> > time skew, but they put them in various places, depending on the daemon
> > and distro in question.  It would therefore be good for the kernel to
> > have some clue that there is a problem.
> >
> > The old behavior of marking the TSC unstable is a non-starter because a
> > great many workloads simply cannot tolerate the overheads and latencies
> > of the various non-TSC clocksources.  In addition, NTP-corrected systems
> > sometimes can tolerate significant kernel-space time skew as long as
> > the userspace time sources are within epsilon of atomic-clock time.
> >
> > 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.

The previous "[PATCH clocksource 5/6] clocksource: Suspend the watchdog 
temporarily when high read latency detected" patch, however, should be 
fine. Right?

Cheers,
Longman