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

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Date:   Tue, 11 Jun 2019 17:18:52 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, 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 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,

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).