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

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Date:	Mon, 23 Aug 2010 10:46:03 +0200
From:	jean-philippe francois <jp.francois@...ove.com>
To:	Lin Ming <lin@...g.vg>
Cc:	"H. Peter Anvin" <hpa@...or.com>, linux-acpi@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: System time drifts when processor idle.

2010/8/21 Lin Ming <lin@...g.vg>:
> On Fri, Aug 20, 2010 at 8:55 AM, jean-philippe francois
> <jp.francois@...ove.com> wrote:
>> Hi,
>>
>> Please correct me if I am sending this to the wrong list.
>
> LKML is better for this issue. Added.
>

Thank you. I have been looking at the ML archives, and it seems there
has been issue related to timekeeping lately. So I tested my script
with a decently recent kernel : 2.6.36-rc1.

The timekeeping bug described below still exists.
>>
>> I would like to debug the following issue, which I suspect is related
>> to cpu losing track of time when going idle.
>>
>> When I run the following script :
>>
>> #!/bin/bash
>> # set hwclock to system clock to start
>> # the log file with the same time
>> hwclock -w
>> #start with a fresh logfile
>> touch clocktest.log
>> echo START > clocktest.log
>> while [[ true ]]
>> do
>>    systime=$(date)
>>    hwtime=$(hwclock -r)
>>    echo "$systime | $hwtime" >> clocktest.log
>>    sleep 20
>> done
>>
>> Expected output :
>> On each line, the system time (left) and hardware time (right)
>> should be roughly the same. Time delta between each line should be
>> around 20 seconds
>>
>> Observed output :
>> This is an edited version of the real logfile  so that it fits on one
>> line. the date was removed.
>> START
>> 15:09:43 (UTC+0200) |  15:09:44 CEST -0.500555 secondes
>> 15:10:04 (UTC+0200) |  15:10:05 CEST -1.001562 secondes
>> 15:10:25 (UTC+0200) |  15:10:26 CEST -0.985944 secondes
>> 15:10:46 (UTC+0200) |  15:15:46 CEST -0.032792 secondes <-- 5 minute lag !
>> 15:11:06 (UTC+0200) |  15:16:07 CEST -0.985926 secondes
>> 15:11:27 (UTC+0200) |  15:16:28 CEST -1.001539 secondes
>> 15:11:48 (UTC+0200) |  15:16:49 CEST -0.985969 secondes
>> 15:12:09 (UTC+0200) |  15:17:10 CEST -0.985977 secondes
>> 15:12:30 (UTC+0200) |  15:17:31 CEST -1.001544 secondes
>> 15:12:51 (UTC+0200) |  15:17:52 CEST -0.985920 secondes
>> 15:13:12 (UTC+0200) |  15:18:13 CEST -0.985939 secondes
>> 15:13:33 (UTC+0200) |  15:18:34 CEST -1.001581 secondes
>> 15:13:54 (UTC+0200) |  15:18:55 CEST -0.985917 secondes
>> 15:14:15 (UTC+0200) |  15:29:15 CEST -0.079685 secondes <-- add
>> another 10 minute
>> 15:14:35 (UTC+0200) |  15:29:36 CEST -1.001568 secondes
>> 15:14:56 (UTC+0200) |  15:29:57 CEST -0.985952 secondes
>>
>> As you can see, after 20 minutes without touching the computer, the
>> system time is late by 15 minutes.
>> This was tested on ubuntu 10.04 with a 2.6.32  "vanilla" kernel where
>> "vanilla" means ubuntu packaged
>> upstream kernel  closest to the distribution shipped kernel.
>>
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