linux-kernel - rcu_sched stall detected, but no state dump

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Date:	Wed, 10 Dec 2014 13:52:02 +0100 (CET)
From:	Miroslav Benes <mbenes@...e.cz>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: rcu_sched stall detected, but no state dump

Hi,

today I came across RCU stall which was correctly detected, but there is 
no state dump. This is a bit suspicious, I think. 

This is the output in serial console:

[  105.727003] INFO: rcu_sched detected stalls on CPUs/tasks:
[  105.727003]  (detected by 0, t=21002 jiffies, g=3269, c=3268, q=138)
[  105.727003] INFO: Stall ended before state dump start
[  168.732006] INFO: rcu_sched detected stalls on CPUs/tasks:
[  168.732006]  (detected by 0, t=84007 jiffies, g=3269, c=3268, q=270)
[  168.732006] INFO: Stall ended before state dump start
[  231.737003] INFO: rcu_sched detected stalls on CPUs/tasks:
[  231.737003]  (detected by 0, t=147012 jiffies, g=3269, c=3268, q=388)
[  231.737003] INFO: Stall ended before state dump start
[  294.742003] INFO: rcu_sched detected stalls on CPUs/tasks:
[  294.742003]  (detected by 0, t=210017 jiffies, g=3269, c=3268, q=539)
[  294.742003] INFO: Stall ended before state dump start
[  357.747003] INFO: rcu_sched detected stalls on CPUs/tasks:
[  357.747003]  (detected by 0, t=273022 jiffies, g=3269, c=3268, q=693)
[  357.747003] INFO: Stall ended before state dump start
[  420.752003] INFO: rcu_sched detected stalls on CPUs/tasks:
[  420.752003]  (detected by 0, t=336027 jiffies, g=3269, c=3268, q=806)
[  420.752003] INFO: Stall ended before state dump start
...

It can be reproduced by trivial code attached to this mail (infinite 
loop in kernel thread created in kernel module). I have CONFIG_PREEMPT=n. 
The kernel thread is scheduled on the same CPU which causes soft lockup 
(reliably detected when lockup detector is on). There is certainly RCU 
stall, but I would expect a state dump. Is this an expected behaviour? 
Maybe I overlooked some config option, don't know.

I tested 3.18 and also next-20141210. If it is improper behaviour I could 
try to find a good kernel release and bisect it.

Best regards,
--
Miroslav Benes
SUSE Labs
View attachment "kthread_mod.c" of type "TEXT/x-c++src" (604 bytes)