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

[Paolo Bonzini]

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.

Paolo