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. 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?
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