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    SubjectRe: 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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