Srivatsa,
On Fri, 12 Apr 2013, Srivatsa S. Bhat wrote:
> On 04/12/2013 02:17 AM, Thomas Gleixner wrote:
> >> +
> >> + /*
> >> + * Wait for p->on_rq to be reset to 0, to ensure that the per-cpu
> >> + * migration thread (which belongs to the stop_task sched class)
> >> + * doesn't run until the cpu is actually onlined and the thread is
> >> + * unparked.
> >> + */
> >> + if (!wait_task_inactive(p, TASK_INTERRUPTIBLE))
> >> + WARN_ON(1);
> >
> > Yay, we rely on TASK_INTERRUPTIBLE state with a task which already has
> > references outside the creation code.
>
> I doubt that. We have not even onlined the CPU, how would any else even
> _know_ that we created this kthread??
The problem is not only at the thread creation time. We have the same
issue at offline/online and there we have a reference to that very
thread.
> >> /**
> >> * kthread_unpark - unpark a thread created by kthread_create().
> >> * @k: thread created by kthread_create().
> >> @@ -337,18 +357,29 @@ void kthread_unpark(struct task_struct *k)
> >> struct kthread *kthread = task_get_live_kthread(k);
> >>
> >> if (kthread) {
> >> + /*
> >> + * Per-cpu kthreads such as ksoftirqd can get woken up by
> >> + * other events. So after binding the thread, ensure that
> >> + * it goes off the CPU atleast once, by parking it again.
> >> + * This way, we can ensure that it will run on the correct
> >> + * CPU on subsequent wakeup.
> >> + */
> >> + if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags)) {
> >> + __kthread_bind(k, kthread->cpu);
> >> + clear_bit(KTHREAD_IS_PARKED, &kthread->flags);
> >
> > And how is that f*cking different from the previous code?
> >
> > CPU0 CPU1 CPU2
> > wakeup(T) -> run on CPU1 (last
> > cpu)
> >
> > switch_to(T)
> >
> > __kthread_bind(T, CPU2)
> >
> > clear(KTHREAD_IS_PARKED)
> >
> > leave loop due to !KTHREAD_IS_PARKED
>
> How?? The task will leave the loop only when we clear
> SHOULD_PARK, not when we clear IS_PARKED. So it won't
> leave the loop here. It will cause the kthread to
> perform a fresh complete() for the waiting
> kthread_park()
> on CPU0.
You are right on that, but you tricked me into misreading your
patch. Why? Simply because it is too complex for no reason.
> No, the purpose of clear(IS_PARKED) followed by __kthread_park() is to
> ensure that the task gets *descheduled* atleast once after we did the
> kthread_bind(). And that's because we can't use set_cpus_allowed_ptr() to
> migrate a running kthread (because the kthread could be the migration
> thread). So instead, we use kthread_bind() and depend on sleep->wakeup
> to put the task on the right CPU.
Yeah, it's a nice workaround, though I really prefer a guaranteed well
defined state over this wakeup/sleep/wakeup trickery, which also adds
the additional cost of a wakeup/sleep cycle to the online operation.
> > TASK_PARKED is the very obvious and robust solution which fixes _ALL_
> > of the corner cases, at least as far as I can imagine them. And
> > robustness rules at least in my world.
> >
>
> Yes, I agree that it is robust and has clear semantics. No doubt about
> that. So I won't insist on going with my suggestions.
I'm glad, that we can agree on the robust solution :)
Thanks,
tglx
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