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

[Paul E. McKenney]

On Wed, Feb 01, 2023 at 10:40:56PM -0500, Waiman Long wrote:
> On 2/1/23 14:55, Paul E. McKenney wrote:
> > On Wed, Feb 01, 2023 at 02:26:29PM -0500, Waiman Long wrote:
> > > 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.
> > 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.

Very good, thank you!

							Thanx, Paul