Messages in this thread | | | Date | Tue, 18 Aug 2015 10:49:23 -0700 | Subject | Re: [PATCH 8/9] clocksource: Improve unstable clocksource detection | From | John Stultz <> |
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On Tue, Aug 18, 2015 at 1:38 AM, Thomas Gleixner <tglx@linutronix.de> wrote: > On Mon, 17 Aug 2015, John Stultz wrote: >> On Mon, Aug 17, 2015 at 3:04 PM, Thomas Gleixner <tglx@linutronix.de> wrote: >> > On Mon, 17 Aug 2015, John Stultz wrote: >> > >> >> From: Shaohua Li <shli@fb.com> >> >> >> >> >From time to time we saw TSC is marked as unstable in our systems, while >> > >> > Stray '>' >> > >> >> the CPUs declare to have stable TSC. Looking at the clocksource unstable >> >> detection, there are two problems: >> >> - watchdog clock source wrap. HPET is the most common watchdog clock >> >> source. It's 32-bit and runs in 14.3Mhz. That means the hpet counter >> >> can wrap in about 5 minutes. >> >> - threshold isn't scaled against interval. The threshold is 0.0625s in >> >> 0.5s interval. What if the actual interval is bigger than 0.5s? >> >> >> >> The watchdog runs in a timer bh, so hard/soft irq can defer its running. >> >> Heavy network stack softirq can hog a cpu. IPMI driver can disable >> >> interrupt for a very long time. >> > >> > And they hold off the timer softirq for more than a second? Don't you >> > think that's the problem which needs to be fixed? >> >> Though this is an issue I've experienced (and tried unsuccessfully to >> fix in a more complicated way) with the RT kernel, where high priority >> tasks blocked the watchdog long enough that we'd disqualify the TSC. > > Did it disqualify the watchdog due to HPET wraparounds (5 minutes) or > due to the fixed threshold being applied?
This was years ago, but in my experience, the watchdog false positives were due to HPET wraparounds.
thanks -john
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