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

[Marc.Zyngier@arm.com (Marc Zyngier)]

On Tue, 2011-05-24 at 23:39 +0200, Ingo Molnar wrote:
> * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> 
> > 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.
> 
> Yes, should be possible to just disable it on x86 - no further tricks needed. 
> It's been a long time since i tested that though.

I can confirm this is SMP only. UP is fine. SMP+nosmp locks up as well.

	M.
-- 
Reality is an implementation detail.