linux-kernel - Re: [PATCH v3 2/4] KVM: LAPIC: lapic timer interrupt is injected by posted interrupt

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Date:   Wed, 12 Jun 2019 09:48:58 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v3 2/4] KVM: LAPIC: lapic timer interrupt is injected by
 posted interrupt

On Wed, 12 Jun 2019 at 04:39, Marcelo Tosatti <mtosatti@...hat.com> wrote:
>
> On Tue, Jun 11, 2019 at 08:17:07PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@...cent.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.
> >
> > ~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@...hat.com>
> > Cc: Radim Krčmář <rkrcmar@...hat.com>
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > ---
> >  arch/x86/kvm/lapic.c | 32 +++++++++++++++++++++++++-------
> >  1 file changed, 25 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index e57eeba..020599f 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -133,6 +133,12 @@ inline bool posted_interrupt_inject_timer_enabled(struct kvm_vcpu *vcpu)
> >  }
> >  EXPORT_SYMBOL_GPL(posted_interrupt_inject_timer_enabled);
> >
> > +static inline bool can_posted_interrupt_inject_timer(struct kvm_vcpu *vcpu)
> > +{
> > +     return posted_interrupt_inject_timer_enabled(vcpu) &&
> > +             kvm_hlt_in_guest(vcpu->kvm);
> > +}
>
> Hi Li,

Hi Marcelo,

>
> Don't think its necessary to depend on kvm_hlt_in_guest: Can also use
> exitless injection if the guest is running (think DPDK style workloads
> that busy-spin on network card).
>

There are some discussions here.

https://lkml.org/lkml/2019/6/11/424
https://lkml.org/lkml/2019/6/5/436

Regards,
Wanpeng Li