linux-kernel - Re: [PATCH v3 1/2] KVM: LAPIC: Tune lapic_timer_advance

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Date:   Wed, 11 Sep 2019 17:45:17 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v3 1/2] KVM: LAPIC: Tune lapic_timer_advance_ns smoothly

On 28/08/19 10:19, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> Using a moving average based on per-vCPU lapic_timer_advance_ns to tune 
> smoothly, filter out drastic fluctuation which prevents this before, 
> let's assume it is 10000 cycles.
> 
> 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 | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index e904ff0..181537a 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -69,6 +69,7 @@
>  #define LAPIC_TIMER_ADVANCE_ADJUST_INIT 1000
>  /* step-by-step approximation to mitigate fluctuation */
>  #define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8
> +#define LAPIC_TIMER_ADVANCE_FILTER 10000
>  
>  static inline int apic_test_vector(int vec, void *bitmap)
>  {
> @@ -1484,23 +1485,28 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
>  					      s64 advance_expire_delta)
>  {
>  	struct kvm_lapic *apic = vcpu->arch.apic;
> -	u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns;
> -	u64 ns;
> +	u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns, ns;
> +
> +	if (abs(advance_expire_delta) > LAPIC_TIMER_ADVANCE_FILTER)
> +		/* filter out drastic fluctuations */
> +		return;
>  
>  	/* too early */
>  	if (advance_expire_delta < 0) {
>  		ns = -advance_expire_delta * 1000000ULL;
>  		do_div(ns, vcpu->arch.virtual_tsc_khz);
> -		timer_advance_ns -= min((u32)ns,
> -			timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
> +		timer_advance_ns -= ns;
>  	} else {
>  	/* too late */
>  		ns = advance_expire_delta * 1000000ULL;
>  		do_div(ns, vcpu->arch.virtual_tsc_khz);
> -		timer_advance_ns += min((u32)ns,
> -			timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
> +		timer_advance_ns += ns;
>  	}
>  
> +	timer_advance_ns = (apic->lapic_timer.timer_advance_ns *
> +		(LAPIC_TIMER_ADVANCE_ADJUST_STEP - 1) + advance_expire_delta) /
> +		LAPIC_TIMER_ADVANCE_ADJUST_STEP;
> +
>  	if (abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_DONE)
>  		apic->lapic_timer.timer_advance_adjust_done = true;
>  	if (unlikely(timer_advance_ns > 5000)) {
> 

This looks great.  But instead of patch 2, why not remove
timer_advance_adjust_done altogether?

Paolo