linux-kernel - Re: Cpu-Hotplug and Real-Time

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Date:	Tue, 7 Aug 2007 10:33:36 -0700
From:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Gautham R Shenoy <ego@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	linux-kernel@...r.kernel.org, Dipankar Sarma <dipankar@...ibm.com>,
	Paul E McKenney <paulmck@...ibm.com>
Subject: Re: Cpu-Hotplug and Real-Time

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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