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

[Marcelo Tosatti <mtosatti@redhat.com>]

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...