On 09/06/11 19:53, Frank Rowand wrote:

On 08/26/11 16:55, Paul E. McKenney wrote:

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On Wed, Aug 24, 2011 at 04:58:49PM -0700, Frank Rowand wrote:

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On 08/13/11 03:53, Peter Zijlstra wrote:

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Whee, I can skip release announcements too!

So no the subject ain't no mistake its not, 3.0.1-rt11 is there for the
grabs.

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I have a consistent (every boot) hang on boot.  With a few
hacks to get console output, I get the

  rcu_preempt_state detected stalls on CPUs/tasks

< snip >

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This is an ARM NaviEngine (out of tree, so I also have applied
a series of pages for platform support).

CONFIG_PREEMPT_RT_FULL is set.  Full config is attached.

I have also replicated the problem on the ARM RealView (in tree) and
without the RT patches.

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Hmmm...  The last few that I have seen that looked like this were
due to my messing up rcutorture so that the RCU-boost testing kthreads
ran CPU-bound at real-time priority.

Is it possible that something similar is happening on your system?

                                                        Thanx, Paul

The problem ended up being caused by the allowed cpus mask being set
to all possible cpus for the ksoftirqd on the secondary processors.
So the RCU softirq was never executing on cpu 2.

I'll test the following patch on 3.1 tomorrow.

And the following patch is some clean up for code that is in the
RT patch.  I do not know if this is needed, but after making the
changes in my first patch it seemed reasonable to add some extra
checks here, just in case softirq_check_pending_idle() gets
called in the window before the variable ksoftirqd gets set.

Signed-off-by: Frank Rowand <redacted>
---
 kernel/softirq.c |   25 	14 +	11 -	0 !
 1 file changed, 14 insertions(+), 11 deletions(-)

Index: b/kernel/softirq.c
===================================================================

--- a/kernel/softirq.c
+++ b/kernel/softirq.c

@@ -87,18 +87,21 @@ void softirq_check_pending_idle(void)
 		struct task_struct *tsk;
 
 		tsk = __get_cpu_var(ksoftirqd);
-		/*
-		 * The wakeup code in rtmutex.c wakes up the task
-		 * _before_ it sets pi_blocked_on to NULL under
-		 * tsk->pi_lock. So we need to check for both: state
-		 * and pi_blocked_on.
-		 */
-		raw_spin_lock(&tsk->pi_lock);
+		if (tsk) {
+			/*
+			 * The wakeup code in rtmutex.c wakes up the task
+			 * _before_ it sets pi_blocked_on to NULL under
+			 * tsk->pi_lock. So we need to check for both: state
+			 * and pi_blocked_on.
+			 */
+			raw_spin_lock(&tsk->pi_lock);
+
+			if (!tsk->pi_blocked_on &&
+			    !(tsk->state == TASK_RUNNING))
+				warnpending = 1;
 
-		if (!tsk->pi_blocked_on && !(tsk->state == TASK_RUNNING))
-			warnpending = 1;