Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested

Paul E. McKenney
2018-07-18T20:14:44Z

On Wed, Jul 18, 2018 at 09:41:05PM +0200, David Woodhouse wrote:
>
>
> On Wed, 2018-07-18 at 09:37 -0700, Paul E. McKenney wrote:
> > On Wed, Jul 18, 2018 at 06:01:51PM +0200, David Woodhouse wrote:
> > >
> > > On Wed, 2018-07-18 at 08:36 -0700, Paul E. McKenney wrote:
> > > >
> > > > And I finally did get some near misses from an earlier commit, so we
> > > > should consider your patch to be officially off the hook.
> > >
> > > Yay, I like it when it's not my fault. I'll redo it with the ifdef
> > > CONFIG_NO_HZ_FULL.
> >
> > Hey, I didn't say it wasn't your fault, only that it -officially- wasn't
> > your fault.  ;-)
>
> I can live with being innocent until proven guilty.
>
> > >
> > > What should it do for the !CONFIG_NO_HZ_FULL case? The existing call in
> > > guest_enter_irqoff() clearly wasn't actually doing the right thing
> > > anyway, hence the need for the need_resched() patch in $SUBJECT... so
> > > should I just leave it doing nothing in guest_enter_irqoff()?
> >
> > One starting point would be the combination of your patch and my
> > patch, with -rcu commit IDs and diff below.  But yes, it needs to be
> > !CONFIG_NO_HZ_FULL.  And no, I am not at all confident that I actually
> > found all the places needing change in the core code, so this needs some
> > serious review both by the KVM guys and the NO_HZ_FULL guys.
>
> Right, that looks fairly much like the version I'd ended up with. So my
> question was...
>
> > --- a/include/linux/context_tracking.h
> > +++ b/include/linux/context_tracking.h
> > @@ -118,12 +118,12 @@ static inline void guest_enter_irqoff(void)
> >    * one time slice). Lets treat guest mode as quiescent state, just like
> >    * we do with user-mode execution.
> >    */
>
> ...if we change this to something like...
>
> #ifdef CONFIG_NO_HZ_FULL
> > + rcu_kvm_enter();
> #else
> > if (!context_tracking_cpu_is_enabled())
> > rcu_virt_note_context_switch(smp_processor_id());
> #endif
>
> ... do you actually want me to keep the #else case there? It blatantly
> wasn't working anyway for us, perhaps because the condition was false?
> That's why I started fixing need_resched() in the first place, and that
> fix ought to cover whatever this call to rcu_virt_note_context_switch()
> was supposed to be doing?

My thought would be something like this:

if (context_tracking_cpu_is_enabled())
rcu_kvm_enter();
else
rcu_virt_note_context_switch(smp_processor_id());

The reason I believe that this is the right approach is that even when
you have CONFIG_NO_HZ_FULL=y, some CPUs will still be nohz_full=n CPUs,
so you don't want to take the extra overhead on those CPUs.

But I could easily be confused here, so I am adding Frederic for his
thoughts.

Thanx, Paul