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

[Paul E. McKenney]

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