netdev - Re: Soft-Lockup/Race in networking in 2.6.31-rc1+195 ( possibly?caused by netem)

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Message-ID: <20090703120301.GD4847@ff.dom.local>
Date:	Fri, 3 Jul 2009 12:03:01 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Andres Freund <andres@...razel.de>
Cc:	Arun R Bharadwaj <arun@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stephen Hemminger <shemminger@...tta.com>,
	netdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Soft-Lockup/Race in networking in 2.6.31-rc1+195 (
	possibly?caused by netem)

On Fri, Jul 03, 2009 at 01:26:21PM +0200, Andres Freund wrote:
> On Friday 03 July 2009 08:12:13 Jarek Poplawski wrote:
> > On Fri, Jul 03, 2009 at 03:31:31AM +0200, Andres Freund wrote:
> > ...
> >
> > > Ok. I finally see the light. I bisected the issue down to
> > > eea08f32adb3f97553d49a4f79a119833036000a :  timers: Logic to move non
> > > pinned timers
> > >
> > > Disabling timer migration like provided in the earlier commit stops the
> > > issue from occuring.
> > >
> > > That it is related to timers is sensible in the light of my findings,
> > > that I could trigger the issue only when using delay in netem - that is
> > > the codepath using qdisc_watchdog...
> >
> > Andres, thanks for your work and time. It saved me a lot of searching,
> > because I wasn't able to trigger this on my old box.
> Thanks. It allowed me to go through some of my remaining paperwork ;-)
> 
> Does anybody of you have an idea where the problem actually resides? 

Do you mean possibly broken timers are not enough?

> qdisc_watchdog_schedule looks innocent enough for my uneducated eyes - and the 
> patch/infrastructure from Arun goes over my head...
> I will happily test some ideas/patches.
> 
> Aside from that - is the whole PSCHED_TICKS2NS/PSCHED_NS2TICKS conversion 
> business purely backward compatibility?

The whole PSCHED_ conversion was to get finer resolution without
breaking backward compatibility, I hope.;-)

Jarek P.
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