Re: [PATCHv2] KVM: optimize apic interrupt delivery

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 11, 2012 at 11:04:59PM +0300, Avi Kivity wrote:
> On 09/11/2012 08:13 PM, Paul E. McKenney wrote:
> > > Is there a risk of DOS if RCU is delayed while
> > > lots of memory is queued up in this way?
> > > If yes is this a generic problem with kfree_rcu
> > > that should be addressed in core kernel?
> >
> > There is indeed a risk.  The kfree_rcu() implementation cannot really
> > decide what to do here, especially given that it is callable with irqs
> > disabled.
> >
> > The usual approach is to keep a per-CPU counter and count it down from
> > some number for each kfree_rcu().  When it reaches zero, invoke
> > synchronize_rcu() as well as kfree_rcu(), and then reset it to the
> > "some number" mentioned above.
> >
> > In theory, I could create an API that did this.  In practice, I have no
> > idea how to choose the number -- much depends on the size of the object
> > being freed, for example.
> 
> Perhaps approach it from the other direction?  If we are under memory
> pressure, start synchronize_rcu()ing, much like the shrinker operates.
> 

Tricky ...

For now, how about we call synchronize_rcu_expedited in kvm and call it a day?
Also has an advantage that apic map is guaranteed to be in sync
with guest - while it seems that it's already correct as is,
synchronous operation is way simpler.

We can add a tracepoint so that we can detect it if this starts
happening a lot for some guest.

> 
> -- 
> I have a truly marvellous patch that fixes the bug which this
> signature is too narrow to contain.