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

[Gleb Natapov <gleb@redhat.com>]

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.