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Subject: Re: [PATCH 1/2] sched: Fix "divide error: 0000" in
 find_busiest_group
From: Peter Zijlstra <peterz@infradead.org>
To: Mike Galbraith <efault@gmx.de>
Cc: Terry Loftin <terry.loftin@hp.com>, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@elte.hu>, Bob Montgomery <bob.montgomery@hp.com>
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Date: Wed, 20 Jul 2011 04:29:51 +0200
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On Wed, 2011-07-20 at 04:26 +0200, Mike Galbraith wrote:
> On Tue, 2011-07-19 at 23:17 +0200, Peter Zijlstra wrote:
> > On Tue, 2011-07-19 at 14:58 -0600, Terry Loftin wrote:
> > > Correct the protection expression in update_cpu_power() to avoid setting
> > > rq->cpu_power to zero.
> > 
> > Firstly you fail to mention what kernel this is again, secondly this
> > should never happen in the first place, so this fix is wrong. At best it
> > papers over another bug.
> > 
> > > Signed-off-by: Terry Loftin <terry.loftin@hp.com>
> > > Signed-off-by: Bob Montgomery <bob.montgomery@hp.com>
> > > ---
> > > diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> > > index 0c26e2d..9c50020 100644
> > > --- a/kernel/sched_fair.c
> > > +++ b/kernel/sched_fair.c
> > > @@ -2549,7 +2549,7 @@ static void update_cpu_power(struct sched_domain *sd, int cpu)
> > >  	power *= scale_rt_power(cpu);
> > >  	power >>= SCHED_LOAD_SHIFT;
> > > 
> > > -	if (!power)
> > > +	if ((u32)power == 0)
> > >  		power = 1;
> > > 
> > >  	cpu_rq(cpu)->cpu_power = power;
> 
> I put that (and a bunch more protection+warnings) in an enterprise
> kernel so it would not explode, but would gather some data.  The entire
> world has been utterly silent, except for a gaggle of POWER7 boxen,
> which manage to convince scale_rt_power() to return negative values.
> 
> Turning on PRINTK_TIME made these boxen go silent.  A printk with
> timestamps, which doesn't happen, hides the problem.  Tilt.

Did those kernels contain the scale_rt_power() hunk from commit
aa483808516ca5cacfa0e5849691f64fec25828e? Venki thought that might cure
sure woes, but since we never could reproduce...