Re: [PATCH 4/4] Convert irq notifiers lists to RCU locking.

Gleb Natapov
2009-07-13T13:53:02Z

On Mon, Jul 13, 2009 at 09:40:01AM -0400, Gregory Haskins wrote:
> Gleb Natapov wrote:
> > On Mon, Jul 13, 2009 at 09:26:21AM -0400, Gregory Haskins wrote:
> >
> >> Gleb Natapov wrote:
> >>
> >>> On Mon, Jul 13, 2009 at 04:02:56PM +0300, Michael S. Tsirkin wrote:
> >>>
> >>>
> >>>> On Sun, Jul 12, 2009 at 03:03:53PM +0300, Gleb Natapov wrote:
> >>>>
> >>>>
> >>>>> Use RCU locking for mask/ack notifiers lists.
> >>>>>
> >>>>> Signed-off-by: Gleb Natapov <gleb@redhat.com>
> >>>>> ---
> >>>>> virt/kvm/irq_comm.c | 20 +++++++++++---------
> >>>>> 1 files changed, 11 insertions(+), 9 deletions(-)
> >>>>>
> >>>>> diff --git a/virt/kvm/irq_comm.c b/virt/kvm/irq_comm.c
> >>>>> index 5dde1ef..ba3a115 100644
> >>>>> --- a/virt/kvm/irq_comm.c
> >>>>> +++ b/virt/kvm/irq_comm.c
> >>>>> @@ -179,18 +179,18 @@ void kvm_notify_acked_irq(struct kvm *kvm, unsigned irqchip, unsigned pin)
> >>>>> break;
> >>>>> }
> >>>>> }
> >>>>> - rcu_read_unlock();
> >>>>>
> >>>>> - hlist_for_each_entry(kian, n, &kvm->irq_ack_notifier_list, link)
> >>>>> + hlist_for_each_entry_rcu(kian, n, &kvm->irq_ack_notifier_list, link)
> >>>>> if (kian->gsi == gsi)
> >>>>> kian->irq_acked(kian);
> >>>>> + rcu_read_unlock();
> >>>>> }
> >>>>>
> >>>>> void kvm_register_irq_ack_notifier(struct kvm *kvm,
> >>>>> struct kvm_irq_ack_notifier *kian)
> >>>>> {
> >>>>> mutex_lock(&kvm->irq_lock);
> >>>>> - hlist_add_head(&kian->link, &kvm->irq_ack_notifier_list);
> >>>>> + hlist_add_head_rcu(&kian->link, &kvm->irq_ack_notifier_list);
> >>>>> mutex_unlock(&kvm->irq_lock);
> >>>>> }
> >>>>>
> >>>>> @@ -198,8 +198,9 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
> >>>>> struct kvm_irq_ack_notifier *kian)
> >>>>> {
> >>>>> mutex_lock(&kvm->irq_lock);
> >>>>> - hlist_del_init(&kian->link);
> >>>>> + hlist_del_init_rcu(&kian->link);
> >>>>> mutex_unlock(&kvm->irq_lock);
> >>>>> + synchronize_rcu();
> >>>>> }
> >>>>>
> >>>>>
> >>>> This is done under kvm->lock still, which means the lock might be held
> >>>> potentially for a very long time. Can synchronize_rcu be moved out of
> >>>> this lock?
> >>>>
> >>>>
> >>>>
> >>> Only if kvm_free_assigned_device() will be moved out of this lock.
> >>> Device de-assignment is not very frequent event though. How long do you
> >>> think it may be held? KVM RCU read sections are very brief.
> >>>
> >>>
> >> Note that the delay imposed by the barrier is not only related to the
> >> length of the critical section. The barrier blocks until the next grace
> >> period, and depending on the type of RCU you are using and your config
> >> options, this could be multiple milliseconds.
> >>
> >> I am not saying that this is definitely a problem for your design. I
> >> am just pointing out that the length of the KVM-RCU read section is only
> >>
> > Yeah I understand that other RCU read section may introduce delays too.
> > The question is how big the delay may be.
>
> I think you are misunderstanding me. The read-side CS is not a
> significant factor here so I am not worried about concurrent read-side
> CS causing a longer delay. What I am saying is that the grace period of
> your RCU subsystem is the dominant factor in the equation here, and this
> may be several milliseconds.
>
How is the "grace period" is determined? Isn't it just means "no cpus is
in RCU read section anymore"?

> > I don't think multiple
> > milliseconds delay in device de-assignment is a big issue though.
> >
>
> I would tend to agree with you. It's not fast path.
>
> I only brought this up because I saw your design being justified
> incorrectly: you said "KVM RCU read sections are very brief", but that
> is not really relevant to Michael's point. I just want to make sure
> that the true impact is understood.
>
> Kind Regards,
> -Greg
>
>

--
Gleb.