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

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Message-ID: <20071004191921.GA24011@dreamland.darkstar.lan>
Date:	Thu, 4 Oct 2007 21:19:21 +0200
From:	Luca Tettamanti <kronos.it@...il.com>
To:	Frans Pop <elendil@...net.nl>
Cc:	Willy Tarreau <w@....eu>, LKML <linux-kernel@...r.kernel.org>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	"Alexander E. Patrakov" <patrakov@....usu.ru>
Subject: Re: Decreasing stime running confuses top (was: top displaying
	9999% CPU usage)

Il Thu, Oct 04, 2007 at 01:32:44AM +0200, Frans Pop ha scritto: 
> On Wednesday 03 October 2007, you wrote:
> > On Wed, Oct 03, 2007 at 09:27:41PM +0200, Frans Pop wrote:
> > > 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...
> > >
> > > So, is it normal that stime decreases sometimes or a kernel bug?
> > > /me expects the last...
> >
> > Let me guess... Dual core AMD64 ?
> 
> Nope: Intel(R) Pentium(R) D CPU 3.20GHz

I just notice the same thing here, with a Core2 Duo (which is supposed
to have synced TSCs) and working HPET.

> The following may well be relevant.
> With 2.6.22 and early 2.6.23-rc kernels (rc3-rc6) I often had this in my
> kernel log (see http://lkml.org/lkml/2007/9/16/45):
>    checking TSC synchronization [CPU#0 -> CPU#1]:
>    Measured 248 cycles TSC warp between CPUs, turning off TSC clock.
>    Marking TSC unstable due to check_tsc_sync_source failed

I don't see this though, TSCs are always syncronized between the 2
cores.

Luca
-- 
Ligabue canta: "Tutti vogliono viaggiare in primaaaa..."
Io ci ho provato e dopo un chilometro ho fuso il motore e bruciato
la frizione...
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