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

    * 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.

    Thanks,

    Ingo


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