Messages in this thread | | | Date | Wed, 1 Feb 2023 23:10:43 +0800 | From | Feng Tang <> | Subject | Re: [PATCH v2 clocksource 6/7] clocksource: Verify HPET and PMTMR when TSC unverified |
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Hi Thomas,
On Wed, Feb 01, 2023 at 11:24:14AM +0100, 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.
Yes, this is a valid concern. Waiman, Paul and I discussed this and had some proposal to handle this side effect, like only watchdoging HPET/ACPI-PM timer for a short period of time in this case. https://lore.kernel.org/lkml/20221227183819.GI4001@paulmck-ThinkPad-P17-Gen-1/ My bad that I didn't follow up as my proposed code looked ugly as bringing more complexsities. Does the idea of setting a watchdog time limit sound fine to you?
Thanks, Feng
> Patently bad idea, no cookies for you! > > Thanks, > > tglx
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