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

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Date:	Fri, 3 Jul 2009 13:26:21 +0200
From:	Andres Freund <andres@...razel.de>
To:	Jarek Poplawski <jarkao2@...il.com>
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 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? 
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?

Andres
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