Date: Mon, 31 Aug 2015 23:47:52 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Shaohua Li <shli@fb.com>
cc: John Stultz <john.stultz@linaro.org>, lkml <linux-kernel@vger.kernel.org>,
        Prarit Bhargava <prarit@redhat.com>,
        Richard Cochran <richardcochran@gmail.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Ingo Molnar <mingo@kernel.org>, Clark Williams <williams@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 8/9] clocksource: Improve unstable clocksource
 detection
In-Reply-To: <20150831211233.GA1413758@devbig257.prn2.facebook.com>
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References: <1439844063-7957-1-git-send-email-john.stultz@linaro.org> <1439844063-7957-9-git-send-email-john.stultz@linaro.org> <alpine.DEB.2.11.1508172326020.3873@nanos> <CALAqxLWOzkbbFeAUKrE9B=O786vuJVmWQdagbfxkpLJDm2mXwQ@mail.gmail.com>
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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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