linux-kernel - Decreasing stime running confuses top (was: top displaying 9999% CPU usage)

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Date:	Wed, 3 Oct 2007 21:27:41 +0200
From:	Frans Pop <elendil@...net.nl>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>,
	"Alexander E. Patrakov" <patrakov@....usu.ru>
Subject: Decreasing stime running confuses top (was: top displaying 9999% CPU usage)

On Wednesday 03 October 2007, you wrote:
> On Wed, 3 Oct 2007, Ilpo Järvinen wrote:
> > On Wed, 3 Oct 2007, Frans Pop wrote:
> > > The only change is in 2 consecutive columns: "2911 502" -> "2912
> > > 500". Is processor usage calculated from those? Can someone explain
> > > how?
> >
> > The latter seems to be utime ...decreasing. No wonder if arithmetics
> > will give strange results (probably top is using unsigned delta?)...
>
> Hmm, minor miscounting from my side, stime seems more appropriate...

Here is a series showing utime and stime for kontact over 2 minutes.

Values were obtained using (identical values removed):
$ while true; do awk '{print $14" "$15}' /proc/5269/stat; sleep 1; done | ts

Oct 03 21:17:12 12220 1593
Oct 03 21:17:18 12221 1594
Oct 03 21:17:26 12222 1593  <--
Oct 03 21:17:34 12223 1594
Oct 03 21:17:43 12224 1594
Oct 03 21:17:51 12224 1595
Oct 03 21:17:59 12225 1596
Oct 03 21:18:07 12226 1595  <--
Oct 03 21:18:15 12227 1596
Oct 03 21:18:18 12228 1596
Oct 03 21:18:22 12229 1595  <--
Oct 03 21:18:31 12230 1596
Oct 03 21:18:39 12230 1597
Oct 03 21:18:44 12231 1597
Oct 03 21:18:48 12232 1596  <--
Oct 03 21:18:56 12233 1597
Oct 03 21:19:04 12234 1596  <--
Oct 03 21:19:11 12235 1597

So, is it normal that stime decreases sometimes or a kernel bug?
/me expects the last...
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