Subject: Re: localed stuck in recent 3.18 git in copy_net_ns?

From: Jay Vosburgh <[email-masked]>
Date: Fri, 24 Oct 2014 15:02:04 -0700

Paul E. McKenney <[email-masked]> wrote:

>On Sat, Oct 25, 2014 at 12:25:57AM +0300, Yanko Kaneti wrote:
>> On Fri-10/24/14-2014 11:32, Paul E. McKenney wrote:
>> > On Fri, Oct 24, 2014 at 08:35:26PM +0300, Yanko Kaneti wrote:
>> > > On Fri-10/24/14-2014 10:20, Paul E. McKenney wrote:
>
>[ . . . ]
>
>> > > > Well, if you are feeling aggressive, give the following patch a spin.
>> > > > I am doing sanity tests on it in the meantime.
>> > > 
>> > > Doesn't seem to make a difference here
>> > 
>> > OK, inspection isn't cutting it, so time for tracing.  Does the system
>> > respond to user input?  If so, please enable rcu:rcu_barrier ftrace before
>> > the problem occurs, then dump the trace buffer after the problem occurs.
>> 
>> Sorry for being unresposive here, but I know next to nothing about tracing
>> or most things about the kernel, so I have some cathing up to do.
>> 
>> In the meantime some layman observations while I tried to find what exactly
>> triggers the problem.
>> - Even in runlevel 1 I can reliably trigger the problem by starting libvirtd
>> - libvirtd seems to be very active in using all sorts of kernel facilities
>>   that are modules on fedora so it seems to cause many simultaneous kworker 
>>   calls to modprobe
>> - there are 8 kworker/u16 from 0 to 7
>> - one of these kworkers always deadlocks, while there appear to be two
>>   kworker/u16:6 - the seventh
>
>Adding Tejun on CC in case this duplication of kworker/u16:6 is important.
>
>>   6 vs 8 as in 6 rcuos where before they were always 8
>> 
>> Just observations from someone who still doesn't know what the u16
>> kworkers are..
>
>Could you please run the following diagnostic patch?  This will help
>me see if I have managed to miswire the rcuo kthreads.  It should
>print some information at task-hang time.

	I can give this a spin after the ftrace (now that I've got
CONFIG_RCU_TRACE turned on).

	I've got an ftrace capture from unmodified -net, it looks like
this:

    ovs-vswitchd-902   [000] ....   471.778441: rcu_barrier: rcu_sched Begin cpu -1 remaining 0 # 0
    ovs-vswitchd-902   [000] ....   471.778452: rcu_barrier: rcu_sched Check cpu -1 remaining 0 # 0
    ovs-vswitchd-902   [000] ....   471.778452: rcu_barrier: rcu_sched Inc1 cpu -1 remaining 0 # 1
    ovs-vswitchd-902   [000] ....   471.778453: rcu_barrier: rcu_sched OnlineNoCB cpu 0 remaining 1 # 1
    ovs-vswitchd-902   [000] ....   471.778453: rcu_barrier: rcu_sched OnlineNoCB cpu 1 remaining 2 # 1
    ovs-vswitchd-902   [000] ....   471.778453: rcu_barrier: rcu_sched OnlineNoCB cpu 2 remaining 3 # 1
    ovs-vswitchd-902   [000] ....   471.778454: rcu_barrier: rcu_sched OnlineNoCB cpu 3 remaining 4 # 1
    ovs-vswitchd-902   [000] ....   471.778454: rcu_barrier: rcu_sched Inc2 cpu -1 remaining 4 # 2
         rcuos/0-9     [000] ..s.   471.793150: rcu_barrier: rcu_sched CB cpu -1 remaining 3 # 2
         rcuos/1-18    [001] ..s.   471.793308: rcu_barrier: rcu_sched CB cpu -1 remaining 2 # 2

	I let it sit through several "hung task" cycles but that was all
there was for rcu:rcu_barrier.

	I should have ftrace with the patch as soon as the kernel is
done building, then I can try the below patch (I'll start it building
now).

	-J

>							Thanx, Paul
>
>------------------------------------------------------------------------
>
>rcu: Dump no-CBs CPU state at task-hung time
>
>Strictly diagnostic commit for rcu_barrier() hang.  Not for inclusion.
>
>Signed-off-by: Paul E. McKenney <[email-masked]>
>
>diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
>index 0e5366200154..34048140577b 100644
>--- a/include/linux/rcutiny.h
>+++ b/include/linux/rcutiny.h
>@@ -157,4 +157,8 @@ static inline bool rcu_is_watching(void)
> 
> #endif /* #else defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) */
> 
>+static inline void rcu_show_nocb_setup(void)
>+{
>+}
>+
> #endif /* __LINUX_RCUTINY_H */
>diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
>index 52953790dcca..0b813bdb971b 100644
>--- a/include/linux/rcutree.h
>+++ b/include/linux/rcutree.h
>@@ -97,4 +97,6 @@ extern int rcu_scheduler_active __read_mostly;
> 
> bool rcu_is_watching(void);
> 
>+void rcu_show_nocb_setup(void);
>+
> #endif /* __LINUX_RCUTREE_H */
>diff --git a/kernel/hung_task.c b/kernel/hung_task.c
>index 06db12434d72..e6e4d0f6b063 100644
>--- a/kernel/hung_task.c
>+++ b/kernel/hung_task.c
>@@ -118,6 +118,7 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
> 		" disables this message.\n");
> 	sched_show_task(t);
> 	debug_show_held_locks(t);
>+	rcu_show_nocb_setup();
> 
> 	touch_nmi_watchdog();
> 
>diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
>index 240fa9094f83..6b373e79ce0e 100644
>--- a/kernel/rcu/rcutorture.c
>+++ b/kernel/rcu/rcutorture.c
>@@ -1513,6 +1513,7 @@ rcu_torture_cleanup(void)
> {
> 	int i;
> 
>+	rcu_show_nocb_setup();
> 	rcutorture_record_test_transition();
> 	if (torture_cleanup_begin()) {
> 		if (cur_ops->cb_barrier != NULL)
>diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>index 927c17b081c7..285b3f6fb229 100644
>--- a/kernel/rcu/tree_plugin.h
>+++ b/kernel/rcu/tree_plugin.h
>@@ -2699,6 +2699,31 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
> 
> #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
> 
>+void rcu_show_nocb_setup(void)
>+{
>+#ifdef CONFIG_RCU_NOCB_CPU
>+	int cpu;
>+	struct rcu_data *rdp;
>+	struct rcu_state *rsp;
>+
>+	for_each_rcu_flavor(rsp) {
>+		pr_alert("rcu_show_nocb_setup(): %s nocb state:\n", rsp->name);
>+		for_each_possible_cpu(cpu) {
>+			if (!rcu_is_nocb_cpu(cpu))
>+				continue;
>+			rdp = per_cpu_ptr(rsp->rda, cpu);
>+			pr_alert("%3d: %p l:%p n:%p %c%c%c\n",
>+				 cpu,
>+				 rdp, rdp->nocb_leader, rdp->nocb_next_follower,
>+				 ".N"[!!rdp->nocb_head],
>+				 ".G"[!!rdp->nocb_gp_head],
>+				 ".F"[!!rdp->nocb_follower_head]);
>+		}
>+	}
>+#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
>+}
>+EXPORT_SYMBOL_GPL(rcu_show_nocb_setup);
>+
> /*
>  * An adaptive-ticks CPU can potentially execute in kernel mode for an
>  * arbitrarily long period of time with the scheduling-clock tick turned
>

---
	-Jay Vosburgh, [email-masked]