Subject: Re: [PATCH] KVM: X86: Reduce the overhead when lapic_timer_advance is
 disabled
To: Wanpeng Li <kernellwp@gmail.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Wanpeng Li <wanpeng.li@hotmail.com>
References: <1512116110-8355-1-git-send-email-wanpeng.li@hotmail.com>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <217b8854-a6d8-1c84-a137-8ddcf051de41@redhat.com>
Date: Fri, 1 Dec 2017 09:35:57 +0100
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On 01/12/2017 09:15, Wanpeng Li wrote:
> When I run ebizzy in a 32 vCPUs guest on a 32 pCPUs Xeon box, I can observe 
> ~8000 kvm_wait_lapic_expire CurAvg/s through kvm_stat tool even if the advance 
> tscdeadline hrtimer expiration is disabled. Each call to wait_lapic_expire() 
> will consume ~70 cycles when a timer fires since apic_timer_expire() will 
> set expired_tscdeadline and then wait_lapic_expire() will do some caculation 
> before bailing out. So total ~175us per second is lost on this 3.2Ghz machine. 
> This patch reduces the overhead by skipping the function wait_lapic_expire() 
> when lapic_timer_advance is disabled.
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
> ---
>  arch/x86/kvm/x86.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

Paolo

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 50c8361..d1abf13 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6994,7 +6994,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	}
>  
>  	trace_kvm_entry(vcpu->vcpu_id);
> -	wait_lapic_expire(vcpu);
> +	if (lapic_timer_advance_ns)
> +		wait_lapic_expire(vcpu);
>  	guest_enter_irqoff();
>  
>  	if (unlikely(vcpu->arch.switch_db_regs)) {
>