Re: Possible nohz-full/RCU issue in arm64 KVM

[Paolo Bonzini]

On 12/17/21 17:07, Paul E. McKenney wrote:
> > rcu_note_context_switch() is a point-in-time notification; it's not strictly
> > necessary, but it may improve performance a bit by avoiding unnecessary IPIs
> > from the RCU subsystem.
> >
> > There's no benefit from doing it when you're back from the guest, because at
> > that point the CPU is just running normal kernel code.
>
> Do scheduling-clock interrupts from guest mode have the "user" parameter
> set?  If so, that would keep RCU happy.

No, thread is in supervisor mode.  But after every interrupt (timer tick 
or anything), one of three things can happen:

* KVM will go around the execution loop and invoke 
rcu_note_context_switch() again

* or KVM will go back to user space

* or the thread will be preempted

and either will keep RCU happy as far as I understand.

Paolo