Re: Can RCU stall lead to hard lockups?

Paul E. McKenney
2018-02-03T20:50:41Z

On Fri, Feb 02, 2018 at 05:44:30PM -0600, Serge E. Hallyn wrote:
> Quoting Paul E. McKenney (paulmck@linux.vnet.ibm.com):
> > 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.
>
> Hi - would this also be relevant to 4.9-stable and 4.4-stable, or
> has something elsewhere changed after 4.9 that actually triggers this?

As far as I can tell, slow console lines have been prone to RCU CPU stall
warnings for a very long time.

Thanx, Paul

> thanks,
> -serge
>
> > 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;
> > >
>