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

From: Wanpeng Li <kernellwp@gmail.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: "Paolo Bonzini" <pbonzini@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
	"Radim Krčmář" <rkrcmar@redhat.com>
Subject: Re: [PATCH v4 2/5] KVM: LAPIC: inject lapic timer interrupt by posted interrupt
Date: Wed, 19 Jun 2019 08:36:06 +0800	[thread overview]
Message-ID: <CANRm+Cz0v1VfDaCCWX+5RzCusTV7g9Hwr+OLGDRijeyqFx=Kzw@mail.gmail.com> (raw)
In-Reply-To: <20190618133541.GA3932@amt.cnet>

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...

Here some new discussions:
https://lkml.org/lkml/2019/6/13/1423
https://lkml.org/lkml/2019/6/13/1420