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

Marcelo Tosatti
2019-06-18T13:36:27Z

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...