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

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Message-ID: <alpine.DEB.2.11.1508312328220.15006@nanos>
Date:	Mon, 31 Aug 2015 23:47:52 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Shaohua Li <shli@...com>
cc:	John Stultz <john.stultz@...aro.org>,
	lkml <linux-kernel@...r.kernel.org>,
	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>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 8/9] clocksource: Improve unstable clocksource
 detection

On Mon, 31 Aug 2015, Shaohua Li wrote:
> > The HPET wraps interval is 0xffffffff / 100000000 = 42.9s
> > 
> > tsc interval is (0x481250b45b - 0x219e6efb50) / 2200000000 = 75s
> > 
> > 32.1 + 42.9 = 75
> > 
> > The example shows hpet wraps, while tsc is marked unstable
> 
> Thomas & John,
> Is this data enough to prove TSC unstable issue can be triggered by HPET
> wrap? I can resend the patch with the data included.

Well, it's enough data to prove:

 - that keeping a VM off the CPU for 75 seconds is insane.

 - that emulating the HPET with 100MHz shortens the HPET wraparound by
   a factor of 7 compared to real hardware. With a realist HPET
   frequency you have about 300 seconds.

   Who though that using 100MHz HPET frequency is a brilliant idea?

So we should add crappy heuristics to the watchdog just to workaround
virt insanities? I'm not convinced.

Thanks,

	tglx
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