linux-kernel - Re: System time drifts when processor idle.

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Date:	Fri, 27 Aug 2010 17:10:19 -0700
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	jean-philippe francois <jp.francois@...ove.com>,
	Lin Ming <lin@...g.vg>, "H. Peter Anvin" <hpa@...or.com>,
	linux-acpi@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	jslaby@...e.cz
Subject: Re: System time drifts when processor idle.

On Fri, Aug 27, 2010 at 11:11 AM, john stultz <johnstul@...ibm.com> wrote:
> On Fri, 2010-08-27 at 16:12 +0200, jean-philippe francois wrote:
>> My Timekeeping bug is still present, here is an updated script and log.
>> I am willing to make test, but I don't know what kind of debugging
>> info is needed.
>>
>> cat /sys/devices/system/clocksource/clocksource0/available_clocksource
>> hpet acpi_pm
>> cat /sys/devices/system/clocksource/clocksource0/current_clocksource
>> hpet
>
> Huh. hpet was not what I would have expected.
>
>
> So first, two experiments:
>
> 1) Does booting with "clock=acpi_pm" cause the issue to disappear?
>
> 2) Does booting with "nohz=off" cause the issue to disappear?
>
>
> Venkatesh: You have any experience with HPETs that halt in idle?
>

No. Haven't seen anything like this before with HPET.

The jump was

system | hardware(RTC)
15:48:43 | ven. 27 août 2010 15:48:44 CEST  -0.985908 secondes
15:49:04 | ven. 27 août 2010 15:54:04 CEST  -0.032800 secondes

We lost ~300 seconds and from dmesg
hpet0: 3 comparators, 64-bit 14.318180 MHz counter

Which is close to 2^32 HPET ticks. So, looks like we have some 32 bit
wraparound somewhere..

Thanks,
Venki
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