Re: Random panic in load_balance() with 3.16-rc

Peter Zijlstra
2014-07-23T14:25:09Z

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.