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SubjectRe: [PATCH v3 2/4] KVM: LAPIC: lapic timer interrupt is injected by posted interrupt
On Tue, Jun 11, 2019 at 08:17:07PM +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.
>
> ~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>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.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,

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).

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