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?