On Tue, Aug 07, 2007 at 07:13:36PM +0400, Oleg Nesterov wrote:
> On 08/07, Gautham R Shenoy wrote:
> >
> > After some debugging, I saw that the hang occured because
> > the high prio process was stuck in a loop doing yield() inside
> > wait_task_inactive(). Description follows:
> >
> > Say a high-prio task (A) does a kthread_create(B),
> > followed by a kthread_bind(B, cpu1). At this moment,
> > only cpu0 is online.
> >
> > Now, immediately after being created, B would
> > do a
> > complete(&create->started) [kernel/kthread.c: kthread()],
> > before scheduling itself out.
> >
> > This complete() will wake up kthreadd, which had spawned B.
> > It is possible that during the wakeup, kthreadd might preempt B.
> > Thus, B is still on the runqueue, and not yet called schedule().
> >
> > kthreadd, will inturn do a
> > complete(&create->done); [kernel/kthread.c: create_kthread()]
> > which will wake up the thread which had called kthread_create().
> > In our case it's task A, which will run immediately, since its priority
> > is higher.
> >
> > A will now call kthread_bind(B, cpu1).
> > kthread_bind(), calls wait_task_inactive(B), to ensures that
> > B has scheduled itself out.
> >
> > B is still on the runqueue, so A calls yield() in wait_task_inactive().
> > But since A is the task with the highest prio, scheduler schedules it
> > back again.
> >
> > Thus B never gets to run to schedule itself out.
> > A loops waiting for B to schedule out leading to system hang.
>
> As for kthread_bind(), I think wait_task_inactive+set_task_cpu is just
> an optimization, and easy to "fix":
>
> --- kernel/kthread.c 2007-07-28 16:58:17.000000000 +0400
> +++ /proc/self/fd/0 2007-08-07 18:56:54.248073547 +0400
> @@ -166,10 +166,7 @@ void kthread_bind(struct task_struct *k,
> WARN_ON(1);
> return;
> }
> - /* Must have done schedule() in kthread() before we set_task_cpu */
> - wait_task_inactive(k);
> - set_task_cpu(k, cpu);
> - k->cpus_allowed = cpumask_of_cpu(cpu);
> + set_cpus_allowed(current, cpumask_of_cpu(cpu));
> }
> EXPORT_SYMBOL(kthread_bind);
>
Not sure whether set_cpus_allowed() will work here. Looks like, it needs the
CPU to be online during the call and in kthread_bind() case CPU may be offline.
Thanks,
Venki
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