Re: Can RCU stall lead to hard lockups?

[Paul E. McKenney]

On Tue, Jan 09, 2018 at 06:11:14AM -0800, Tejun Heo wrote:
> Hello, Paul.
> 
> On Mon, Jan 08, 2018 at 08:24:25PM -0800, Paul E. McKenney wrote:
> > > I don't know the RCU code at all but it *looks* like the first CPU is
> > > taking a sweet while flushing printk buffer while holding a lock (the
> > > console is IPMI serial console, which faithfully emulates 115200 baud
> > > rate), and everyone else seems stuck waiting for that spinlock in
> > > rcu_check_callbacks().
> > > 
> > > Does this sound possible?
> > 
> > 115200 baud?  Ouch!!!  That -will- result in trouble from console
> > printing, and often also in RCU CPU stall warnings.
> 
> It could even be slower than 115200, and we occassionally see RCU
> stall warnings caused by printk storms, for example, while the kernel
> is trying to dump a lot of info after an OOM.  That's an issue we
> probably want to improve from printk side; however, they don't usually
> lead to NMI hard lockup detector kicking in and crashing the machine,
> which is the peculiarity here.
> 
> Hmmm... show_state_filter(), the function which dumps all task
> backtraces, share a similar problem and it avoids it by explicitly
> calling touch_nmi_watchdog().  Maybe we can do something like the
> following from RCU too?

If this fixes things for you, I would welcome such a patch.

							Thanx, Paul

> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index db85ca3..3c4c4d3 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -561,8 +561,14 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
>  	}
>  	t = list_entry(rnp->gp_tasks->prev,
>  		       struct task_struct, rcu_node_entry);
> -	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
> +	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
> +		touch_nmi_watchdog();
> +		/*
> +		 * We could be printing a lot of these messages while
> +		 * holding a spinlock.  Avoid triggering hard lockup.
> +		 */
>  		sched_show_task(t);
> +	}
>  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  }
> 
> @@ -1678,6 +1684,12 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
>  	char *ticks_title;
>  	unsigned long ticks_value;
> 
> +	/*
> +	 * We could be printing a lot of these messages while holding a
> +	 * spinlock.  Avoid triggering hard lockup.
> +	 */
> +	touch_nmi_watchdog();
> +
>  	if (rsp->gpnum == rdp->gpnum) {
>  		ticks_title = "ticks this GP";
>  		ticks_value = rdp->ticks_this_gp;
>