linux-kernel - Re: Clocksource tsc unstable (delta = -8589361717 ns) in current git

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Message-ID: <20101027142622.GA1555@arch.trippelsdorf.de>
Date:	Wed, 27 Oct 2010 16:26:22 +0200
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	john stultz <johnstul@...ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...64.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...ux.intel.com" <hpa@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andreas Herrmann <andreas.herrmann3@....com>
Subject: Re: Clocksource tsc unstable (delta = -8589361717 ns) in current
 git

On Tue, Oct 26, 2010 at 12:18:56PM -0700, john stultz wrote:
> On Tue, 2010-10-26 at 17:48 +0200, Thomas Gleixner wrote:
> > On Tue, 26 Oct 2010, Markus Trippelsdorf wrote:
> > > On Tue, Oct 26, 2010 at 03:18:43PM +0200, Borislav Petkov wrote:
> > > > otherwise, I don't see anything strange in your dmesg. Unless tglx has a
> > > > better idea, I'd ask you to bisect it. 5618 changesets shouldn't be that
> > > > much but I don't know, if the issue appears every several hours it could
> > > > still be tedious.
> > > 
> > > That would be a several week long process, as the issue appears ~ every
> > > 2 days here on this machine running 24/7.
> > 
> > There is only a single commit in that area post 2.6.36:
> > 
> >       8af3c153baf95374eff20a37f00c59a295b52756 
> > 
> > But I have a hard time how this should make this happen. John ?
> 
> Yea, that one doesn't look connected to me.
> 
> There have been a few cases that I've seen where we can get false
> positives for bad TSCs due to the watchdog clocksource having problems
> (or the clocksource watchdog thread getting delayed for such a long time
> the watchdog clocksource wraps and we then can't validly compare the two
> - although this would be hard to trigger with non-rt kernels).

I think the remark above points in the right direction, because the
mysterious slowness happened again today (and this time on a vanilla git
kernel). But this time no "Clocksource tsc unstable" message appeared in
the kernel log. My system recovered by itself and after 2 minutes of
slowness it was usable again. (The symptoms were the same as desribed in
my first mail: Second long delays when typing and switching from X to
the console took many seconds.)

Is there anything I can do to pinpoint the cause of this slowdown should
it happen again? (Maybe a perf timechart?)

-- 
Markus
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