Messages in this thread | | | Date | Fri, 11 Dec 2020 02:40:31 +0100 | From | Frederic Weisbecker <> | Subject | Re: NOHZ tick-stop error: Non-RCU local softirq work is pending |
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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...
Thanks.
> > Thanx, Paul > > > Thanks!
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