Re: [BUG] "sched: Remove rq->lock from the first half of ttwu()" locks up on ARM

Peter Zijlstra
2011-05-24T21:29:15Z

On Tue, 2011-05-24 at 19:13 +0100, Marc Zyngier wrote:
> Peter,
>
> I've experienced all kind of lock-ups on ARM SMP platforms recently, and
> finally tracked it down to the following patch:
>
> e4a52bcb9a18142d79e231b6733cabdbf2e67c1f [sched: Remove rq->lock from the first half of ttwu()].
>
> Even on moderate load, the machine locks up, often silently, and
> sometimes with a few messages like:
> INFO: rcu_preempt_state detected stalls on CPUs/tasks: { 0} (detected by 1, t=12002 jiffies)
>
> Another side effect of this patch is that the load average is always 0,
> whatever load I throw at the system.
>
> Reverting the sched changes up to that patch (included) gives me a
> working system again, which happily survives parallel kernel
> compilations without complaining.
>
> My knowledge of the scheduler being rather limited, I haven't been able
> to pinpoint the exact problem (though it probably have something to do
> with __ARCH_WANT_INTERRUPTS_ON_CTXSW being defined on ARM). The enclosed
> patch somehow papers over the load average problem, but the system ends
> up locking up anyway:

Hurm.. I'll try and make x86 use __ARCH_WANT_INTERRUPTS_ON_CTXSW, IIRC
Ingo once said that that is possible and try to see if I can reproduce.
No clear ideas atm.

Thanks for reporting.