On Mon, 25 Apr 2011 14:49:33 "Paul E. McKenney" wrote:
> On Mon, Apr 25, 2011 at 02:30:02PM -0700, Linus Torvalds wrote:
> > 2011/4/25 Bruno PrÃmont <bonbons@xxxxxxxxxxxxxxxxx>:
> > >
> > > Between 1-slabinfo and 2-slabinfo some values increased (a lot) while a few
> > > ones did decrease. Don't know which ones are RCU-affected and which ones are
> > > not.
> >
> > It really sounds as if the tiny-rcu kthread somehow just stops
> > handling callbacks. The ones that keep increasing do seem to be all
> > rcu-free'd (but I didn't really check).
> >
> > The thing is shown as running:
> >
> > root 6 0.0 0.0 0 0 ? R 22:14 0:00 \_
> > [rcu_kthread]
> >
> > but nothing seems to happen and the CPU time hasn't increased at all.
> >
> > I dunno. Makes no sense to me, but yeah, I'm definitely blaming
> > tiny-rcu. Paul, any ideas?
>
> So the only ways I know for something to be runnable but not run on
> a uniprocessor are:
>
> 1. The CPU is continually busy with higher-priority work.
> This doesn't make sense in this case because the system
> is idle much of the time.
>
> 2. The system is hibernating. This doesn't make sense, otherwise
> "ps" wouldn't run either.
>
> Any others ideas on how the heck a process can get into this state?
> (I have thus far been completely unable to reproduce it.)
>
> The process in question has a loop in rcu_kthread() in kernel/rcutiny.c.
> This loop contains a wait_event_interruptible(), waits for a global flag
> to become non-zero.
>
> It is awakened by invoke_rcu_kthread() in that same file, which
> simply sets the flag to 1 and does a wake_up(), all with hardirqs
> disabled.
>
> Hmmm... One "hail mary" patch below. What it does is make rcu_kthread
> run at normal priority rather than at real-time priority. This is
> not for inclusion -- it breaks RCU priority boosting. But well worth
> trying.
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
> index 0c343b9..4551824 100644
> --- a/kernel/rcutiny.c
> +++ b/kernel/rcutiny.c
> @@ -314,11 +314,15 @@ EXPORT_SYMBOL_GPL(rcu_barrier_sched);
> */
> static int __init rcu_spawn_kthreads(void)
> {
> +#if 0
> struct sched_param sp;
> +#endif
>
> rcu_kthread_task = kthread_run(rcu_kthread, NULL, "rcu_kthread");
> +#if 0
> sp.sched_priority = RCU_BOOST_PRIO;
> sched_setscheduler_nocheck(rcu_kthread_task, SCHED_FIFO, &sp);
> +#endif
> return 0;
> }
> early_initcall(rcu_spawn_kthreads);
I will give that patch a shot on Wednesday evening (European time) as I
wont have enough time in front of the affected box until then to do any
deeper testing. (same for trying to out with the other -rc kernels as
suggested by Mike)
Though I will use the few minutes I have this evening to try to fetch
kernel traces of running tasks with sysrq+t which may eventually give
us a hint at where rcu_thread is stuck/waiting.
Bruno
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