linux-kernel - Re: [PATCH/RFC] timer: fix deadlock on cpu hotplug

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Date:	Wed, 22 Sep 2010 10:37:07 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Tejun Heo <tj@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFC] timer: fix deadlock on cpu hotplug

On Tue, Sep 21, 2010 at 05:40:18PM +0200, Peter Zijlstra wrote:
> On Tue, 2010-09-21 at 17:36 +0200, Tejun Heo wrote:
> > I think this is the second time we're seeing deadlock during cpu down
> > due to RT throttling and timer problem.  The rather delicate
> > dependency there makes me somewhat nervous.  If possible, I think it
> > would be better if we can simply turn the RT throttling off when
> > cpu_stop kicks in.  It's intended to be a mechanism to monopolize all
> > CPU cycles to begin with.  Would that be difficult?
> 
> I've wanted to pull the whole migration thread out from SCHED_FIFO for a
> while. Doing that is probably the easiest thing.

Something like this?

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 4372ccb..854fd57 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -291,7 +291,6 @@ repeat:
 static int __cpuinit cpu_stop_cpu_callback(struct notifier_block *nfb,
 					   unsigned long action, void *hcpu)
 {
-	struct sched_param param = { .sched_priority = MAX_RT_PRIO - 1 };
 	unsigned int cpu = (unsigned long)hcpu;
 	struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
 	struct task_struct *p;
@@ -304,7 +303,6 @@ static int __cpuinit cpu_stop_cpu_callback(struct notifier_block *nfb,
 				   cpu);
 		if (IS_ERR(p))
 			return NOTIFY_BAD;
-		sched_setscheduler_nocheck(p, SCHED_FIFO, &param);
 		get_task_struct(p);
 		stopper->thread = p;
 		break;

...gets stuck nearly immediatly on cpu hotplug stress if the machine is
doing anything but idling around.
I was too lazy to figure out why it got stuck. I'm afraid that with such
a change a new class of bugs will appear.
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