Re: [PATCH 1/4] Move irq routing data structure to rcu locking

Gleb Natapov
2009-07-13T13:15:39Z

On Mon, Jul 13, 2009 at 09:01:33AM -0400, Gregory Haskins wrote:
> Gleb Natapov wrote:
> > Signed-off-by: Gleb Natapov <gleb@redhat.com>
> > ---
> > include/linux/kvm_host.h | 2 +-
> > virt/kvm/irq_comm.c | 55 +++++++++++++++++++++-------------------------
> > virt/kvm/kvm_main.c | 1 -
> > 3 files changed, 26 insertions(+), 32 deletions(-)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index f54a0d3..6756b3e 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -161,7 +161,7 @@ struct kvm {
> >
> > struct mutex irq_lock;
> > #ifdef CONFIG_HAVE_KVM_IRQCHIP
> > - struct list_head irq_routing; /* of kvm_kernel_irq_routing_entry */
> > + struct kvm_kernel_irq_routing_entry *irq_routing;
> > struct hlist_head mask_notifier_list;
> > #endif
> >
> > diff --git a/virt/kvm/irq_comm.c b/virt/kvm/irq_comm.c
> > index 7af18b8..b2fa3f6 100644
> > --- a/virt/kvm/irq_comm.c
> > +++ b/virt/kvm/irq_comm.c
> > @@ -148,7 +148,8 @@ int kvm_set_irq(struct kvm *kvm, int irq_source_id, int irq, int level)
> > * IOAPIC. So set the bit in both. The guest will ignore
> > * writes to the unused one.
> > */
> > - list_for_each_entry(e, &kvm->irq_routing, link)
> > + rcu_read_lock();
> > + for (e = rcu_dereference(kvm->irq_routing); e && e->set; e++) {
> >
>
> Hi Gleb,
> I haven't had a chance to fully digest and review these patches, but
> one thing I did notice is that you seem to be converting from a list to
> an open-coded structure. I am just curious why you made this design
> decision instead of using the RCU variant of list?
>
It is not scary "open-coded structure" it's just an array :) As I responded
to Michael the idea is to move msis out of irq_routing, make the array
much smaller and either use gsi as an index in the array or use hash table
instead looping over all entries. For now I can justify array as more
cache friendly data structure as we scan it linearly.

> Regards,
> -Greg
>
> > if (e->gsi == irq) {
> > int r = e->set(e, kvm, sig_level);
> > if (r < 0)
> > @@ -156,6 +157,8 @@ int kvm_set_irq(struct kvm *kvm, int irq_source_id, int irq, int level)
> >
> > ret = r + ((ret < 0) ? 0 : ret);
> > }
> > + }
> > + rcu_read_unlock();
> > return ret;
> > }
> >
> > @@ -168,12 +171,15 @@ void kvm_notify_acked_irq(struct kvm *kvm, unsigned irqchip, unsigned pin)
> >
> > trace_kvm_ack_irq(irqchip, pin);
> >
> > - list_for_each_entry(e, &kvm->irq_routing, link)
> > + rcu_read_lock();
> > + for (e = rcu_dereference(kvm->irq_routing); e && e->set; e++) {
> > if (e->irqchip.irqchip == irqchip &&
> > e->irqchip.pin == pin) {
> > gsi = e->gsi;
> > break;
> > }
> > + }
> > + rcu_read_unlock();
> >
> > hlist_for_each_entry(kian, n, &kvm->arch.irq_ack_notifier_list, link)
> > if (kian->gsi == gsi)
> > @@ -264,19 +270,11 @@ void kvm_fire_mask_notifiers(struct kvm *kvm, int irq, bool mask)
> > kimn->func(kimn, mask);
> > }
> >
> > -static void __kvm_free_irq_routing(struct list_head *irq_routing)
> > -{
> > - struct kvm_kernel_irq_routing_entry *e, *n;
> > -
> > - list_for_each_entry_safe(e, n, irq_routing, link)
> > - kfree(e);
> > -}
> > -
> > void kvm_free_irq_routing(struct kvm *kvm)
> > {
> > - mutex_lock(&kvm->irq_lock);
> > - __kvm_free_irq_routing(&kvm->irq_routing);
> > - mutex_unlock(&kvm->irq_lock);
> > + /* Called only during vm destruction. Nobody can use the pointer
> > + at this stage */
> > + kfree(kvm->irq_routing);
> > }
> >
> > static int setup_routing_entry(struct kvm_kernel_irq_routing_entry *e,
> > @@ -326,43 +324,40 @@ int kvm_set_irq_routing(struct kvm *kvm,
> > unsigned nr,
> > unsigned flags)
> > {
> > - struct list_head irq_list = LIST_HEAD_INIT(irq_list);
> > - struct list_head tmp = LIST_HEAD_INIT(tmp);
> > - struct kvm_kernel_irq_routing_entry *e = NULL;
> > + struct kvm_kernel_irq_routing_entry *new, *old;
> > unsigned i;
> > int r;
> >
> > + /* last element is left zeroed and indicates the end of the array */
> > + new = kzalloc(sizeof(*new) * (nr + 1), GFP_KERNEL);
> > +
> > + if (!new)
> > + return -ENOMEM;
> > +
> > for (i = 0; i < nr; ++i) {
> > r = -EINVAL;
> > if (ue->gsi >= KVM_MAX_IRQ_ROUTES)
> > goto out;
> > if (ue->flags)
> > goto out;
> > - r = -ENOMEM;
> > - e = kzalloc(sizeof(*e), GFP_KERNEL);
> > - if (!e)
> > - goto out;
> > - r = setup_routing_entry(e, ue);
> > + r = setup_routing_entry(new + i, ue);
> > if (r)
> > goto out;
> > ++ue;
> > - list_add(&e->link, &irq_list);
> > - e = NULL;
> > }
> >
> > mutex_lock(&kvm->irq_lock);
> > - list_splice(&kvm->irq_routing, &tmp);
> > - INIT_LIST_HEAD(&kvm->irq_routing);
> > - list_splice(&irq_list, &kvm->irq_routing);
> > - INIT_LIST_HEAD(&irq_list);
> > - list_splice(&tmp, &irq_list);
> > + old = kvm->irq_routing;
> > + rcu_assign_pointer(kvm->irq_routing, new);
> > mutex_unlock(&kvm->irq_lock);
> >
> > + synchronize_rcu();
> > +
> > r = 0;
> > + new = old;
> >
> > out:
> > - kfree(e);
> > - __kvm_free_irq_routing(&irq_list);
> > + kfree(new);
> > return r;
> > }
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index cf20dc1..24013b4 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -945,7 +945,6 @@ static struct kvm *kvm_create_vm(void)
> > if (IS_ERR(kvm))
> > goto out;
> > #ifdef CONFIG_HAVE_KVM_IRQCHIP
> > - INIT_LIST_HEAD(&kvm->irq_routing);
> > INIT_HLIST_HEAD(&kvm->mask_notifier_list);
> > #endif
> >
> >
>
>

--
Gleb.