linux-kernel - Re: [PATCH 1/2] sched: Fix "divide error: 0000" in find_busiest

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Message-ID: <1311128780.7789.12.camel@marge.simson.net>
Date:	Wed, 20 Jul 2011 04:26:20 +0200
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Terry Loftin <terry.loftin@...com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Bob Montgomery <bob.montgomery@...com>
Subject: Re: [PATCH 1/2] sched: Fix "divide error: 0000" in
 find_busiest_group

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@...com>
> > Signed-off-by: Bob Montgomery <bob.montgomery@...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.

	-Mike

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