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

[Wanpeng Li <kernellwp@gmail.com>]

On Thu, 20 Jun 2019 at 05:04, Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
> Hi Li,
>
> On Wed, Jun 19, 2019 at 08:36:06AM +0800, Wanpeng Li wrote:
> > 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...

https://lkml.org/lkml/2019/6/5/436  "Here you need to check
kvm_halt_in_guest, not kvm_mwait_in_guest, because you need to go
through kvm_apic_expired if the guest needs to be woken up from
kvm_vcpu_block."

I think we can still be woken up from kvm_vcpu_block() if pir is set.

Regards,
Wanpeng Li