From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: linux-kernel@vger.kernel.org
Subject: Re: NOHZ tick-stop error: Non-RCU local softirq work is pending
Date: Fri, 11 Dec 2020 02:40:31 +0100 [thread overview]
Message-ID: <20201211014031.GB580714@lothringen> (raw)
In-Reply-To: <20201211004638.GB2657@paulmck-ThinkPad-P72>
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!
next prev parent reply other threads:[~2020-12-11 1:42 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-18 17:52 NOHZ tick-stop error: Non-RCU local softirq work is pending Paul E. McKenney
2020-11-18 17:54 ` Paul E. McKenney
2020-11-18 20:03 ` Paul E. McKenney
2020-12-10 13:04 ` Frederic Weisbecker
2020-12-10 14:56 ` Frederic Weisbecker
2020-12-10 21:17 ` Paul E. McKenney
2020-12-11 0:15 ` Frederic Weisbecker
2020-12-11 0:46 ` Paul E. McKenney
2020-12-11 1:40 ` Frederic Weisbecker [this message]
2020-12-11 3:24 ` Paul E. McKenney
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