linux-kernel - Re: [PATCH v5 3/3] KVM: LAPIC: Tune lapic_timer_advance

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Message-ID: <879b1950-3ca7-0a53-9e4f-508fd5db4bd4@redhat.com>
Date:   Tue, 17 Sep 2019 19:07:15 +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>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v5 3/3] KVM: LAPIC: Tune lapic_timer_advance_ns smoothly

On 17/09/19 10:16, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> Filter out drastic fluctuation and random fluctuation, remove
> timer_advance_adjust_done altogether, the adjustment would be
> continuous.
> 
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>

Queued, thanks (I renamed the new variable to lapic_timer_advance_dynamic).

Thanks,

Paolo

> ---
>  arch/x86/kvm/lapic.c | 28 ++++++++++++++--------------
>  arch/x86/kvm/lapic.h |  1 -
>  2 files changed, 14 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index dbbe478..323bdca 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -65,7 +65,9 @@
>  #define APIC_BROADCAST			0xFF
>  #define X2APIC_BROADCAST		0xFFFFFFFFul
>  
> -#define LAPIC_TIMER_ADVANCE_ADJUST_DONE 100
> +static bool dynamically_adjust_timer_advance __read_mostly;
> +#define LAPIC_TIMER_ADVANCE_ADJUST_MIN 100
> +#define LAPIC_TIMER_ADVANCE_ADJUST_MAX 5000
>  #define LAPIC_TIMER_ADVANCE_ADJUST_INIT 1000
>  /* step-by-step approximation to mitigate fluctuation */
>  #define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8
> @@ -1485,26 +1487,25 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
>  	u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns;
>  	u64 ns;
>  
> +	/* Do not adjust for tiny fluctuations or large random spikes. */
> +	if (abs(advance_expire_delta) > LAPIC_TIMER_ADVANCE_ADJUST_MAX ||
> +	    abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_MIN)
> +		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/LAPIC_TIMER_ADVANCE_ADJUST_STEP;
>  	} 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/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)) {
> +	if (unlikely(timer_advance_ns > LAPIC_TIMER_ADVANCE_ADJUST_MAX))
>  		timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
> -		apic->lapic_timer.timer_advance_adjust_done = false;
> -	}
>  	apic->lapic_timer.timer_advance_ns = timer_advance_ns;
>  }
>  
> @@ -1524,7 +1525,7 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
>  	if (guest_tsc < tsc_deadline)
>  		__wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);
>  
> -	if (unlikely(!apic->lapic_timer.timer_advance_adjust_done))
> +	if (dynamically_adjust_timer_advance)
>  		adjust_lapic_timer_advance(vcpu, apic->lapic_timer.advance_expire_delta);
>  }
>  
> @@ -2302,13 +2303,12 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int timer_advance_ns)
>  	apic->lapic_timer.timer.function = apic_timer_fn;
>  	if (timer_advance_ns == -1) {
>  		apic->lapic_timer.timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
> -		apic->lapic_timer.timer_advance_adjust_done = false;
> +		dynamically_adjust_timer_advance = true;
>  	} else {
>  		apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> -		apic->lapic_timer.timer_advance_adjust_done = true;
> +		dynamically_adjust_timer_advance = false;
>  	}
>  
> -
>  	/*
>  	 * APIC is created enabled. This will prevent kvm_lapic_set_base from
>  	 * thinking that APIC state has changed.
> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> index 50053d2..2aad7e2 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -35,7 +35,6 @@ struct kvm_timer {
>  	s64 advance_expire_delta;
>  	atomic_t pending;			/* accumulated triggered timers */
>  	bool hv_timer_in_use;
> -	bool timer_advance_adjust_done;
>  };
>  
>  struct kvm_lapic {
>