Re: [PATCHv6 6/8] kvm: only sync when attention bits set

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

On Wed, Jun 13, 2012 at 05:53:36PM -0300, Marcelo Tosatti wrote:
> On Wed, Jun 13, 2012 at 11:35:07AM +0300, Michael S. Tsirkin wrote:
> > On Wed, Jun 13, 2012 at 11:19:24AM +0300, Michael S. Tsirkin wrote:
> > > On Tue, Jun 12, 2012 at 07:27:48PM -0300, Marcelo Tosatti wrote:
> > > > On Sun, Jun 03, 2012 at 10:28:29AM +0300, Michael S. Tsirkin wrote:
> > > > > Commit eb0dc6d0368072236dcd086d7fdc17fd3c4574d4 introduced apic
> > > > > attention bitmask but kvm still syncs lapic unconditionally.
> > > > > As that commit suggested and in anticipation of adding more attention
> > > > > bits, only sync lapic if(apic_attention).
> > > > > 
> > > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > > ---
> > > > >  arch/x86/kvm/x86.c |    3 ++-
> > > > >  1 files changed, 2 insertions(+), 1 deletions(-)
> > > > > 
> > > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > > index be6d549..2f70861 100644
> > > > > --- a/arch/x86/kvm/x86.c
> > > > > +++ b/arch/x86/kvm/x86.c
> > > > > @@ -5388,7 +5388,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> > > > >  	if (unlikely(vcpu->arch.tsc_always_catchup))
> > > > >  		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> > > > >  
> > > > > -	kvm_lapic_sync_from_vapic(vcpu);
> > > > > +	if (unlikely(vcpu->arch.apic_attention))
> > > > > +		kvm_lapic_sync_from_vapic(vcpu);
> > > > 
> > > > void kvm_lapic_sync_from_vapic(struct kvm_vcpu *vcpu)
> > > > {
> > > >         u32 data;
> > > >         void *vapic;
> > > > 
> > > >         if (!test_bit(KVM_APIC_CHECK_VAPIC, &vcpu->arch.apic_attention))
> > > >                 return;
> > > > 
> > > > Please use unlikely more carefully, when a gain is measureable:
> > > > http://lwn.net/Articles/420019/
> > > 
> > > Do we have to measure every single thing?
> > > Sometimes it's obvious: vapic is slow path, isn't it?
> > 
> > Just to clarify the question: I think it's obvious this condition is
> > false more often than true. By how much, depends on the workload.
> > Do you think this is enough to tag this unlikely?
> 
> Depends whether your processor supports flexpriority or not. I don't
> want to argue in favour/against the particular instance
> 
> GCC docs:
> 
> "
> — Built-in Function: long __builtin_expect (long exp, long c)
> 
>     You may use __builtin_expect to provide the compiler with branch
> prediction information. In general, you should prefer to use actual
> profile feedback for this (-fprofile-arcs), as programmers are
> notoriously bad at predicting how their programs actually perform.
> However, there are applications in which this data is hard to collect.
> "
> 
> Lately half of branches in your patches are annotated.

So if I instrument and show that branch is almost never taken that is enough?
This citation does not require measuring the perf impact.