Re: [PATCH v3 5/5] KVM: LAPIC: Optimize timer latency further

[Wanpeng Li <kernellwp@gmail.com>]

On Sat, 18 May 2019 at 03:50, Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Thu, May 16, 2019 at 11:06:20AM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@tencent.com>
> >
> > Advance lapic timer tries to hidden the hypervisor overhead between the
> > host emulated timer fires and the guest awares the timer is fired. However,
> > it just hidden the time between apic_timer_fn/handle_preemption_timer ->
> > wait_lapic_expire, instead of the real position of vmentry which is
> > mentioned in the orignial commit d0659d946be0 ("KVM: x86: add option to
> > advance tscdeadline hrtimer expiration"). There is 700+ cpu cycles between
> > the end of wait_lapic_expire and before world switch on my haswell desktop.
> >
> > This patch tries to narrow the last gap(wait_lapic_expire -> world switch),
> > it takes the real overhead time between apic_timer_fn/handle_preemption_timer
> > and before world switch into consideration when adaptively tuning timer
> > advancement. The patch can reduce 40% latency (~1600+ cycles to ~1000+ cycles
> > on a haswell desktop) for kvm-unit-tests/tscdeadline_latency when testing
> > busy waits.
> >
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Radim Krčmář <rkrcmar@redhat.com>
> > Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> > Cc: Liran Alon <liran.alon@oracle.com>
> > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > ---
> >  arch/x86/kvm/lapic.c   | 3 ++-
> >  arch/x86/kvm/lapic.h   | 2 +-
> >  arch/x86/kvm/svm.c     | 4 ++++
> >  arch/x86/kvm/vmx/vmx.c | 4 ++++
> >  arch/x86/kvm/x86.c     | 3 ---
> >  5 files changed, 11 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index af38ece..63513de 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -1531,7 +1531,7 @@ static inline void adaptive_tune_timer_advancement(struct kvm_vcpu *vcpu,
> >       apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> >  }
> >
> > -void wait_lapic_expire(struct kvm_vcpu *vcpu)
> > +void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
> >  {
> >       struct kvm_lapic *apic = vcpu->arch.apic;
> >       u64 guest_tsc, tsc_deadline;
> > @@ -1553,6 +1553,7 @@ void wait_lapic_expire(struct kvm_vcpu *vcpu)
> >       if (unlikely(!apic->lapic_timer.timer_advance_adjust_done))
> >               adaptive_tune_timer_advancement(vcpu, apic->lapic_timer.advance_expire_delta);
> >  }
> > +EXPORT_SYMBOL_GPL(kvm_wait_lapic_expire);
> >
> >  static void start_sw_tscdeadline(struct kvm_lapic *apic)
> >  {
> > diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> > index 3e72a25..f974a3d 100644
> > --- a/arch/x86/kvm/lapic.h
> > +++ b/arch/x86/kvm/lapic.h
> > @@ -220,7 +220,7 @@ static inline int kvm_lapic_latched_init(struct kvm_vcpu *vcpu)
> >
> >  bool kvm_apic_pending_eoi(struct kvm_vcpu *vcpu, int vector);
> >
> > -void wait_lapic_expire(struct kvm_vcpu *vcpu);
> > +void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu);
> >
> >  bool kvm_intr_is_single_vcpu_fast(struct kvm *kvm, struct kvm_lapic_irq *irq,
> >                       struct kvm_vcpu **dest_vcpu);
> > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > index 6b92eaf..955cfcb 100644
> > --- a/arch/x86/kvm/svm.c
> > +++ b/arch/x86/kvm/svm.c
> > @@ -5638,6 +5638,10 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
> >       clgi();
> >       kvm_load_guest_xcr0(vcpu);
> >
> > +     if (lapic_in_kernel(vcpu) &&
> > +             vcpu->arch.apic->lapic_timer.timer_advance_ns)
>
> Nit: align the two lines of the if statement, doing so makes it easier to
>      differentiate between the condition and execution, e.g.:
>
>         if (lapic_in_kernel(vcpu) &&
>             vcpu->arch.apic->lapic_timer.timer_advance_ns)
>                 kvm_wait_lapic_expire(vcpu);

This can result in checkpatch.pl complain:

WARNING: suspect code indent for conditional statements (8, 24)
#94: FILE: arch/x86/kvm/vmx/vmx.c:6436:
+    if (lapic_in_kernel(vcpu) &&
[...]
+            kvm_wait_lapic_expire(vcpu);

Regards,
Wanpeng Li

>
> > +             kvm_wait_lapic_expire(vcpu);
> > +
> >       /*
> >        * If this vCPU has touched SPEC_CTRL, restore the guest's value if
> >        * it's non-zero. Since vmentry is serialising on affected CPUs, there
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index e1fa935..771d3bf 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -6423,6 +6423,10 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> >
> >       vmx_update_hv_timer(vcpu);
> >
> > +     if (lapic_in_kernel(vcpu) &&
> > +             vcpu->arch.apic->lapic_timer.timer_advance_ns)
> > +             kvm_wait_lapic_expire(vcpu);
>
> Same comment as above.  With those fixed:
>
> Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>
>
> > +
> >       /*
> >        * If this vCPU has touched SPEC_CTRL, restore the guest's value if
> >        * it's non-zero. Since vmentry is serialising on affected CPUs, there
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 4a7b00c..e154f52 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -7903,9 +7903,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >       }
> >
> >       trace_kvm_entry(vcpu->vcpu_id);
> > -     if (lapic_in_kernel(vcpu) &&
> > -         vcpu->arch.apic->lapic_timer.timer_advance_ns)
> > -             wait_lapic_expire(vcpu);
> >       guest_enter_irqoff();
> >
> >       fpregs_assert_state_consistent();
> > --
> > 2.7.4
> >