Re: Cpu-Hotplug and Real-Time

[Oleg Nesterov <oleg@tv-sign.ru>]

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);

But I think we have another case. An RT ptracer can share the same CPU
with ptracee. The latter sets TASK_STOPPED, unlocks ->siglock, and takes
a preemption. Ptracer does ptrace_check_attach(), sees TASK_STOPPED, and
yields in wait_task_inactive.

Oleg.