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
