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

David Woodhouse
2018-07-12T12:00:52Z

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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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