linux-kernel - Re: [PATCH RT] Fix RT balancing tasks pulling

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Date:	Tue, 31 Jul 2007 17:18:03 -0700
From:	Josh Triplett <josht@...ux.vnet.ibm.com>
To:	Sébastien Dugué <sebastien.dugue@...l.net>
Cc:	Linux RT Users <linux-rt-users@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jean Pierre Dion <jean-pierre.dion@...l.net>,
	Darren Hart <dvhltc@...ibm.com>,
	john stultz <johnstul@...ibm.com>
Subject: Re: [PATCH RT] Fix RT balancing tasks pulling

On Tue, 2007-07-31 at 13:48 +0200, Sébastien Dugué wrote:
> On Mon, 30 Jul 2007 14:50:34 -0700 Josh Triplett <josht@...ux.vnet.ibm.com> wrote:
> > On Tue, 2007-07-24 at 15:42 +0200, Sébastien Dugué wrote:
> > >   this one-liner fixes a bug in balance_rt_tasks() which sometimes manifests by
> > > having a lower prio task being scheduled while a higher prio task is sitting
> > > waiting on another runqueue.
> > > 
> > >   This is pretty hard to reproduce on low cpu count machines, for example, I
> > > had to have sched_football run in a loop for ~38h before it failed on a dual HT
> > > Xeon box.
> > 
> > Very impressive find, and this does seem to improve things in some
> > cases.
> > 
> > However, with 2.6.22.1-rt8, which includes this patch, I still managed
> > to get some failures after a few hours on an 8-way (quad dual-core) box.
> > Preliminary results so far:
> >     420 Final ball position: 0
> >       2 Final ball position: 1
> > 
> 
>   Darn, I thought it was it :(
> 
>   I had a ~60h run on my box with the very same kernel without a single
> failure.

I think you really need a box with more processors to reproduce these
problems in a reasonable timeframe.  Happy to test any patches you have
to fix RT balancing; please let me know.

>   Do you have any other user RT load running when those failures pop up?

Other than sched_football, I had sshd, bash, and GNU screen,
theoretically all 100% idle (no input to read and no output to display).

- Josh Triplett


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