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

Gregory Haskins
2009-07-13T13:40:11Z

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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.

> 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

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