Re: [PATCH v4 2/5] KVM: LAPIC: inject lapic timer interrupt by posted interrupt

Wanpeng Li
2019-06-19T00:34:53Z

On Tue, 18 Jun 2019 at 21:36, Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
> On Mon, Jun 17, 2019 at 07:24:44PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@tencent.com>
> >
> > Dedicated instances are currently disturbed by unnecessary jitter due
> > to the emulated lapic timers fire on the same pCPUs which vCPUs resident.
> > There is no hardware virtual timer on Intel for guest like ARM. Both
> > programming timer in guest and the emulated timer fires incur vmexits.
> > This patch tries to avoid vmexit which is incurred by the emulated
> > timer fires in dedicated instance scenario.
> >
> > When nohz_full is enabled in dedicated instances scenario, the emulated
> > timers can be offload to the nearest busy housekeeping cpus since APICv
> > is really common in recent years. The guest timer interrupt is injected
> > by posted-interrupt which is delivered by housekeeping cpu once the emulated
> > timer fires.
> >
> > The host admin should fine tuned, e.g. dedicated instances scenario w/
> > nohz_full cover the pCPUs which vCPUs resident, several pCPUs surplus
> > for busy housekeeping, disable mwait/hlt/pause vmexits to keep in non-root
> > mode, ~3% redis performance benefit can be observed on Skylake server.
> >
> > w/o patch:
> >
> > VM-EXIT Samples Samples% Time% Min Time Max Time Avg time
> >
> > EXTERNAL_INTERRUPT 42916 49.43% 39.30% 0.47us 106.09us 0.71us ( +- 1.09% )
> >
> > w/ patch:
> >
> > VM-EXIT Samples Samples% Time% Min Time Max Time Avg time
> >
> > EXTERNAL_INTERRUPT 6871 9.29% 2.96% 0.44us 57.88us 0.72us ( +- 4.02% )
> >
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Radim Krčmář <rkrcmar@redhat.com>
> > Cc: Marcelo Tosatti <mtosatti@redhat.com>
> > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > ---
> > arch/x86/kvm/lapic.c | 33 ++++++++++++++++++++++++++-------
> > arch/x86/kvm/lapic.h | 1 +
> > arch/x86/kvm/vmx/vmx.c | 3 ++-
> > arch/x86/kvm/x86.c | 5 +++++
> > arch/x86/kvm/x86.h | 2 ++
> > include/linux/sched/isolation.h | 2 ++
> > kernel/sched/isolation.c | 6 ++++++
> > 7 files changed, 44 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 87ecb56..9ceeee5 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -122,6 +122,13 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
> > return apic->vcpu->vcpu_id;
> > }
> >
> > +bool posted_interrupt_inject_timer(struct kvm_vcpu *vcpu)
> > +{
> > + return pi_inject_timer && kvm_vcpu_apicv_active(vcpu) &&
> > + kvm_hlt_in_guest(vcpu->kvm);
> > +}
> > +EXPORT_SYMBOL_GPL(posted_interrupt_inject_timer);
>
> Paolo, can you explain the reasoning behind this?
>
> Should not be necessary...