From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gleb Natapov Subject: Re: [PATCH 4/4] Convert irq notifiers lists to RCU locking. Date: Mon, 13 Jul 2009 16:52:57 +0300 Message-ID: <20090713135257.GO28046@redhat.com> References: <1247400233-24243-1-git-send-email-gleb@redhat.com> <1247400233-24243-5-git-send-email-gleb@redhat.com> <20090713130256.GC10402@redhat.com> <20090713131128.GI28046@redhat.com> <4A5B35FD.9090208@gmail.com> <20090713133234.GN28046@redhat.com> <4A5B3931.9080508@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "Michael S. Tsirkin" , avi@redhat.com, "kvm@vger.kernel.org" , paulmck@linux.vnet.ibm.com To: Gregory Haskins Return-path: Received: from mx2.redhat.com ([66.187.237.31]:51725 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755762AbZGMNxC (ORCPT ); Mon, 13 Jul 2009 09:53:02 -0400 Content-Disposition: inline In-Reply-To: <4A5B3931.9080508@gmail.com> Sender: kvm-owner@vger.kernel.org List-ID: 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 > >>>>> --- > >>>>> 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.