Re: NOHZ tick-stop error: Non-RCU local softirq work is pending

From: Paul E. McKenney
Date: Thu Dec 10 2020 - 22:26:32 EST


On Fri, Dec 11, 2020 at 02:40:31AM +0100, Frederic Weisbecker wrote:
> On Thu, Dec 10, 2020 at 04:46:38PM -0800, Paul E. McKenney wrote:
> > > diff --git a/kernel/softirq.c b/kernel/softirq.c
> > > index 09229ad82209..7d558cb7a037 100644
> > > --- a/kernel/softirq.c
> > > +++ b/kernel/softirq.c
> > > @@ -650,7 +650,9 @@ static void run_ksoftirqd(unsigned int cpu)
> > > * We can safely run softirq on inline stack, as we are not deep
> > > * in the task stack here.
> > > */
> > > - __do_softirq();
> > > + do {
> > > + __do_softirq();
> > > + } while (kthread_should_park() && local_softirq_pending());
> > > local_irq_enable();
> > > cond_resched();
> > > return;
> >
> > Huh. I guess that self-propagating timers, RCU callbacks, and the
> > like are non-problems because they cannot retrigger while interrupts
> > are disabled? But can these things reappear just after the
> > local_irq_enable()?
>
> Exactly, unless those things have been disabled in CPU hotplug callbacks
> before smpboot_park_threads() (and we are not too deep in CPU hotplug at that
> stage, cpu_online() is still true), they should be able to raise again
> the softirqs. Yeah I'm pretty sure the above won't be enough.
>
> > In the case of RCU, softirq would need to run on this CPU, which it won't,
> > so we are good in that case. (Any stranded callbacks will be requeued
> > onto some other CPU later in the CPU-hotplug offline processing.)
>
> Ah that's good, so at least we shouldn't worry about pending RCU softirqs
> after ksoftirqd has parked. But I also see at least SCHED_SOFTIRQ and
> TIMER_SOFTIRQ in your list.
>
> Perhaps we should have some flush_softirq() somewhere late in cpu hotplug,
> assuming this is actually necessary and none of these will requeue themselves...

Indeed, let's see if we have a problem before we worry too much about
fixing it. ;-)

I still get the "NOHZ tick-stop error", but more than three times less
often, so definite improvement. One of the console.log file is attached.

Thanx, Paul

Attachment: console1.log.gz
Description: console1.log.gz