Re: [PATCH v3 tip/core/rcu 40/40] rcu: Make non-preemptive schedule be Tasks RCU quiescent state

[Paul E. McKenney]

On Fri, Sep 29, 2017 at 01:44:56PM +0200, Paolo Bonzini wrote:
> On 29/09/2017 12:34, Peter Zijlstra wrote:
> > On Fri, Sep 29, 2017 at 12:01:24PM +0200, Paolo Bonzini wrote:
> >>> Does this mean whenever we get a page fault in a RCU read-side critical
> >>> section, we may hit this?
> >>>
> >>> Could we simply avoid to schedule() in kvm_async_pf_task_wait() if the
> >>> fault process is in a RCU read-side critical section as follow?
> >>>
> >>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> >>> index aa60a08b65b1..291ea13b23d2 100644
> >>> --- a/arch/x86/kernel/kvm.c
> >>> +++ b/arch/x86/kernel/kvm.c
> >>> @@ -140,7 +140,7 @@ void kvm_async_pf_task_wait(u32 token)
> >>>  
> >>>  	n.token = token;
> >>>  	n.cpu = smp_processor_id();
> >>> -	n.halted = is_idle_task(current) || preempt_count() > 1;
> >>> +	n.halted = is_idle_task(current) || preempt_count() > 1 || rcu_preempt_depth();
> >>>  	init_swait_queue_head(&n.wq);
> >>>  	hlist_add_head(&n.link, &b->list);
> >>>  	raw_spin_unlock(&b->lock);
> >>>
> >>> (Add KVM folks and list Cced)
> >>
> >> Yes, that would work.  Mind to send it as a proper patch?
> > 
> > I'm confused, why would we do an ASYNC PF at all here? Thing is, a
> > printk() shouldn't trigger a major fault _ever_. At worst it triggers
> > something like a vmalloc minor fault. And I'm thinking we should not do
> > the whole ASYNC machinery for minor faults.
> 
> Async page faults are page faults _on the host_ side, and you cannot
> control what the host pages out.  Of course the hypervisor filters out
> some cases itself (e.g. IF=0) but in general you could get one at any time.

Just to make sure I am understanding this...  You take a page fault on
the host, and this causes a schedule() on the guest?  Or did I lose the
thread here?

								Thanx, Paul