Re: INFO: task rcu_kthread:6 blocked for more than 120 seconds.

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Jan 12, 2011 at 05:43:00PM +0000, Russell King wrote:
> I'm seeing rcu_kthread get stuck from time to time on ARM Versatile PB
> (ARM926).  Kernel is based upon commit 9e9bc973 (immediately prior to
> the merge of the OMAP tree.)
> 
> ~ # ps aux|grep rcu
> root         6  0.0  0.0      0     0 ?        S    02:31   0:01 [rcu_kthread]
> root       502  9.0  0.3   3740   472 ttyAMA0  S+   02:59   0:00 grep rcu
> ~ # ps aux|grep rcu
> root         6  0.0  0.0      0     0 ?        D    02:31   0:01 [rcu_kthread]
> root       504  0.0  0.3   3740   472 ttyAMA0  S+   03:00   0:00 grep rcu
> ~ # ps aux|grep rcu
> root         6  0.0  0.0      0     0 ?        D    02:31   0:01 [rcu_kthread]
> root       506  0.0  0.3   3740   472 ttyAMA0  S+   03:00   0:00 grep rcu
> ~ # ps aux|grep rcu
> root         6  0.0  0.0      0     0 ?        D    02:31   0:01 [rcu_kthread]
> root       508  0.0  0.3   3740   472 ttyAMA0  S+   03:00   0:00 grep rcu
> ~ # cat /proc/6/wchan
> rcu_kthread
> ~ # INFO: task rcu_kthread:6 blocked for more than 120 seconds.
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> rcu_kthread     D c02c6684     0     6      2 0x00000000
> Backtrace:
> [<c02c6388>] (schedule+0x0/0x354) from [<c00836a0>] (rcu_kthread+0x78/0xf4)
> [<c0083628>] (rcu_kthread+0x0/0xf4) from [<c00633ac>] (kthread+0x88/0x90)
> [<c0063324>] (kthread+0x0/0x90) from [<c004d12c>] (do_exit+0x0/0x5f8)
>  r7:00000013 r6:c004d12c r5:c0063324 r4:c181df28
> no locks held by rcu_kthread/6.

Does the following patch help?

							Thanx, Paul

------------------------------------------------------------------------

commit 70ce43253a4465211a9c51e33d56a417c759a718
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Wed Jan 12 14:18:11 2011 -0800

    rcu: avoid pointless blocked-task warnings
    
    If the RCU callback-processing kthread has nothing to do, it parks in
    a wait_event().  If RCU remains idle for more than two minutes, the
    kernel complains about this.  This commit changes from wait_event()
    to wait_event_interruptible() to prevent the kernel from complaining
    just because RCU is idle.
    
    Reported-by: Russell King <rmk@arm.linux.org.uk>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
index 0344937..0c343b9 100644
--- a/kernel/rcutiny.c
+++ b/kernel/rcutiny.c
@@ -189,7 +189,8 @@ static int rcu_kthread(void *arg)
 	unsigned long flags;
 
 	for (;;) {
-		wait_event(rcu_kthread_wq, have_rcu_kthread_work != 0);
+		wait_event_interruptible(rcu_kthread_wq,
+					 have_rcu_kthread_work != 0);
 		morework = rcu_boost();
 		local_irq_save(flags);
 		work = have_rcu_kthread_work;