Messages in this thread | | | Date | Tue, 6 Feb 2018 20:33:03 -0600 | From | "Serge E. Hallyn" <> | Subject | Re: Can RCU stall lead to hard lockups? |
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On Sat, Feb 03, 2018 at 12:50:32PM -0800, Paul E. McKenney wrote: > 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
Ok, thanks Paul.
Tejun were you going to push this?
> > 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; > > > > > >
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