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

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Date:	Tue, 18 Aug 2015 10:49:23 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
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>
Subject: Re: [PATCH 8/9] clocksource: Improve unstable clocksource detection

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.

thanks
-john
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