linux-kernel - Re: Random panic in load

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Message-ID: <20140723142454.GQ12054@laptop.lan>
Date:	Wed, 23 Jul 2014 16:24:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dietmar Eggemann <dietmar.eggemann@....com>
Cc:	Michel Dänzer <michel@...nzer.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Random panic in load_balance() with 3.16-rc

On Wed, Jul 23, 2014 at 01:30:21PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 23, 2014 at 01:11:10PM +0200, Peter Zijlstra wrote:
> > On Wed, Jul 23, 2014 at 10:45:46AM +0100, Dietmar Eggemann wrote:
> > > Doesn't the picture showing the captured panic reveal more information.
> > > Haven't seen it myself, I just saw Peter's reply to your email
> > 
> > Its a general protection fault from somewhere in load_balance(), I send
> > you the picture.
> > 
> > It would help to get addr2line of the RIP I suppose.
> > 
> > Michel provided a config, so lemme go try and build that, maybe my gcc
> > will generate similar code to his and the function offset is enough
> > clue.
> 
> So the code section says the faulting instruction is:
> 
>   f3 a5
> 
> followed by:
> 
>   48 89 c7 85 50 ff ff 
> 
> or so.
> 
> My compiled code is 'different', the function is shorter, but there's a
> f3 a5 somewhere not too far short of +d7 at +a8. I have (objdump -SD):
> 
>     35a8:       f3 a5                   rep movsl %ds:(%rsi),%es:(%rdi)
> 
>         for_each_cpu_and(i, sched_group_cpus(group), env->cpus) {
>                 unsigned long capacity, capacity_factor, wl;
>                 enum fbq_type rt;
> 
>                 rq = cpu_rq(i);
>     35aa:       48 c7 c1 00 00 00 00    mov    $0x0,%rcx
> 
> And that's the only part that could possibly match.
> 
> That looks like the start of find_busiest_queue(). I'm not entirely sure
> what the rep movsl is operating on, lemme try and figure that out.

Ah, this appears to be load_balance()'s:

	cpumask_copy(cpus, cpu_active_mask);

Which totally doesn't make sense, both src and dst are static storage.
Dst is the most interesting since its per-cpu storage, but still.

No way either of those should generate a #GP. Puzzled.
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