On Wed, Jul 18, 2018 at 01:17:00PM -0700, Paul E. McKenney wrote: > 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. Hmm, actually rcu_user_enter()/rcu_user_exit() are already called upon guest entry/exit :-) Now I must confess the code leading there in __context_tracking_enter/exit is not very obvious but that part applies to both CONTEXT_USER and CONTEXT_GUEST cases. I should probably add a few comments to clarify.
