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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
