lkml.org 
[lkml]   [2012]   [Jun]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCHv6 6/8] kvm: only sync when attention bits set
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.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2012-06-13 23:21    [W:0.862 / U:0.116 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site