On Thu, Jul 12, 2018 at 01:00:42PM +0100, David Woodhouse wrote: > > > On Wed, 2018-07-11 at 14:08 -0700, Paul E. McKenney wrote: > > > > > Also... why in $DEITY's name was the existing > > > rcu_virt_note_context_switch() not actually sufficient? If we had that > > > there, why did we need an additional explicit calls to rcu_all_qs() in > > > the KVM loop, or the more complex fixes to need_resched() which > > > ultimately had the same effect, to avoid ten-second latencies? > > > > My guess is that this was because control passed through the > > rcu_virt_note_context_switch() only once, and then subsequent > > scheduling-clock interrupts bypassed this code. Gah! My guess was instead that the code did a rcu_kvm_enter() going in, but somehow managed to miss the rcu_kvm_exit() going out. But that makes absolutely no sense -- had that happened, rcutorture would likely have screamed bloody murder, loudly and often. No mere near misses! And besides, thus far, -ENOREPRODUCE. :-/ Which indicates that I have an opportunity to improve rcutorture and that this patch was with high probability an innocent bystander. > > But that is just a guess. > > I need to defer to someone who understands the KVM code better than I do. > > I think it's more likely that we just never happened at all. It's > conditional. From the latest patch iteration (see it being removed): > > @@ -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 (!context_tracking_cpu_is_enabled()) > - rcu_virt_note_context_switch(smp_processor_id()); > + rcu_kvm_enter(); > } > > > Given the vmexit overhead, I don't think we can do the currently- > proposed rcu_kvm_enter() thing except for CONFIG_NO_HZ_FULL where it's > really necessary. I'll make that conditional, but probably on the RCU > side. > > Without CONFIG_NO_HZ_FULL, rcu_kvm_exit() can do nothing, and > rcu_kvm_enter() can do rcu_virt_note_context_switch(). > > OK? Makes sense to me! And a big "thank you!" to Christian for testing and analyzing this in a timely fashion!!! Thanx, Paul
