linux-kernel - Re: [PATCH] sched/kthread: Complain loudly when others violate our flags

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Date:	Wed, 28 Sep 2011 02:22:49 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Tejun Heo <htejun@...il.com>, LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] sched/kthread: Complain loudly when others violate our
 flags

On Tue, 27 Sep 2011, Steven Rostedt wrote:
> I looked at the task that it tried to migrate, and it happened to be the
> kworker thread! Then I went into kernel/workqueue.c and found this
> nonsense:
> 
> 	if (bind && !on_unbound_cpu)
> 		kthread_bind(worker->task, gcwq->cpu);
> 	else {
> 		worker->task->flags |= PF_THREAD_BOUND;
> 		if (on_unbound_cpu)
> 			worker->flags |= WORKER_UNBOUND;
> 	}
> 
> Nothing but the scheduler and kthread_bind() has the right to set the
> PF_THREAD_BOUND flag. Especially when the thread IS NOT BOUNDED!!!!!!

Yikes, I somehow missed that gem. :(
 
> I don't go around and stick my hand down your pants to play with your
> flags! Don't stick your hand in ours and play with our flags!
> 
> WTF is the workqueue code setting the PF_THREAD_BOUND flag manually?
> Talk about fragile coupling! You just made this flag meaningless. Don't
> do that.

And looking at the hotplug code in that very file is just making me
more nervous about that abuse.

> Sorry but I just wasted two whole days because of this nonsense and I'm
> not particularly happy about it.
>  
> -- Steve
> 
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> 
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index ec5f472..682a90c 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2233,6 +2233,9 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
>  	if (task_cpu(p) != new_cpu) {
>  		p->se.nr_migrations++;
>  		perf_sw_event(PERF_COUNT_SW_CPU_MIGRATIONS, 1, NULL, 0);
> +		if (WARN_ON(p->flags & PF_THREAD_BOUND))
> +			printk(KERN_WARNING "migrating bounded task %s:%d\n",
> +			       p->comm, p->pid);

That should be a if (WARN(...)) and if triggers we should just refuse
to migrate it. So probably set_task_cpu() is not the proper place for
that. Can we move that check on layer up to avoid it reaching
set_task_cpu() ?

>  	}
>  
>  	__set_task_cpu(p, new_cpu);

And please submit a patch which gets rid of

> 		worker->task->flags |= PF_THREAD_BOUND;

Thanks,

	tglx
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