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

Paul E. McKenney
2018-07-11T21:06:24Z

On Wed, Jul 11, 2018 at 09:19:44PM +0100, David Woodhouse wrote:
> On Wed, 2018-07-11 at 13:17 -0700, Paul E. McKenney wrote:
> > As I understand it, they would like to have their guest run uninterrupted
> > for extended times.  Because rcu_virt_note_context_switch() is a
> > point-in-time quiescent state, it cannot tell RCU about the extended
> > quiescent state.
> >
> > Should we replace the current calls to rcu_virt_note_context_switch()
> > with rcu_kvm_enter() and rcu_kvm_exit()?  Would that be better
> > than the below architecture-by-architecture approach?
>
> Yes it would. I was already starting to mutter about needing the same
> for ARM and POWER. I'll do a v3 (incorporating your fixes too) in the
> morning.
>
> Thanks.
>
> 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.

Thanx, Paul