Re: [PATCH 8/9] clocksource: Improve unstable clocksource detection

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, 18 Aug 2015, John Stultz wrote:
> 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.

Blocking stuff for 5 minutes is insane ....