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

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Date:	Tue, 18 Aug 2015 22:18:09 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	John Stultz <john.stultz@...aro.org>
cc:	lkml <linux-kernel@...r.kernel.org>, Shaohua Li <shli@...com>,
	Prarit Bhargava <prarit@...hat.com>,
	Richard Cochran <richardcochran@...il.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Ingo Molnar <mingo@...nel.org>,
	Clark Williams <williams@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 8/9] clocksource: Improve unstable clocksource
 detection

On Tue, 18 Aug 2015, John Stultz wrote:
> On Tue, Aug 18, 2015 at 12:28 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > On Tue, 18 Aug 2015, John Stultz wrote:
> >> On Tue, Aug 18, 2015 at 1:38 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >> > On Mon, 17 Aug 2015, John Stultz wrote:
> >> >> On Mon, Aug 17, 2015 at 3:04 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >> >> > On Mon, 17 Aug 2015, John Stultz wrote:
> >> >> >
> >> >> >> From: Shaohua Li <shli@...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 ....
> 
> Yea. It was usually due to -RT stress testing, which keept the
> machines busy for quite awhile. But again, if you have machines being
> maxed out with networking load, etc, even for long amounts of time, we
> still want to avoid false positives. Because after the watchdog

The networking softirq does not hog the other softirqs. It has a limit
on processing loops and then goes back to let the other softirqs be
handled. So no, I doubt that heavy networking can cause this. If it
does then we have some other way more serious problems.

I can see the issue with RT stress testing, but not with networking in
mainline.

Thanks,

	tglx
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