On Wed, Jun 13, 2012 at 08:38:46PM -0300, Marcelo Tosatti wrote: > On Thu, Jun 14, 2012 at 12:04:23AM +0300, Michael S. Tsirkin wrote: > > 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@xxxxxxxxxx> > > > > > > > --- > > > > > > > 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. > > Without flexpriority its always taken. OK I will remove this one and keep around the unlikely for the cases that my measurements show occur in less than 1% of cases. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html