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

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Date:	Sat, 21 Aug 2010 03:58:35 +0000
From:	Lin Ming <lin@...g.vg>
To:	jean-philippe francois <jp.francois@...ove.com>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	linux-acpi@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: System time drifts when processor idle.

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.

Lin Ming

>
> 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.
>
> How can I debug and possibly fix this issue ?
> --
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