Re: [PATCH 08/10] rcu: Allow to deactivate nocb on a CPU

Frederic Weisbecker
2020-05-14T22:30:27Z

On Thu, May 14, 2020 at 08:47:07AM -0700, Paul E. McKenney wrote:
> On Thu, May 14, 2020 at 12:45:26AM +0200, Frederic Weisbecker wrote:
> This last seems best to me. The transition from CBLIST_NOT_OFFLOADED
> to CBLIST_OFFLOADING of course needs to be on the CPU in question with
> at least bh disabled. Probably best to be holding rcu_nocb_lock(),
> but that might just be me being overly paranoid.

So that's in the case of offloading, right? Well, I don't think we'd
need to even disable bh nor lock nocb. We just need the current CPU
to see the local update of cblist->offloaded = CBLIST_OFFLOADING
before the kthread is unparked:

cblist->offloaded = CBLIST_OFFLOADING;
/* Make sure subsequent softirq lock nocb */
barrier();
kthread_unpark(rdp->nocb_cb_thread);

Now, although that guarantees that nocb_cb will see CBLIST_OFFLOADING
upon unparking, it's not guaranteed that the nocb_gp will see it on its
next round. Ok so eventually you're right, I should indeed lock nocb...

>
> > > > +static long rcu_nocb_rdp_deoffload(void *arg)
> > > > +{
> > > > + struct rcu_data *rdp = arg;
> > > > +
> > > > + WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
> > > > + __rcu_nocb_rdp_deoffload(rdp);
> > > > +
> > > > + return 0;
> > > > +}
> > >
> > > For example, is the problem caused by invocations of this
> > > rcu_nocb_rdp_deoffload() function?
> >
> > How so?
>
> It looked to me like it wasn't excluding either rcu_barrier() or CPU
> hotplug. It might also not have been pinning onto the CPU in question,
> but that might just be me misremembering. Then again, I didn't see a
> call to it, so maybe its callers set things up appropriately.
>
> OK, I will bite... What is the purpose of rcu_nocb_rdp_deoffload()? ;-)

Ah it's called using work_on_cpu() which launch a workqueue on the
target and waits for completion. And that whole thing is protected
inside the barrier mutex and hotplug.

> Agreed! And I do believe that concurrent callback execution will
> prove better than a possibly indefinite gap in callback execution.

Mutual agreement! :-)

Thanks.