Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH v2 clocksource 6/7] clocksource: Verify HPET and PMTMR when TSC unverified | Date | Wed, 01 Feb 2023 11:24:14 +0100 |
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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!
Thanks,
tglx
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