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

Marcelo Tosatti
2009-07-13T15:56:17Z

On Mon, Jul 13, 2009 at 04:15:34PM +0300, Gleb Natapov wrote:
> 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.

I think its more important to convert to faster search mechanism (the
list walk shows up high in profiling), then convert to RCU?