linux-kernel - Re: [PATCH v2] KVM: LAPIC: Retry tune per-vCPU timer_advance

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Date:   Fri, 5 Jul 2019 17:46:41 +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 v2] KVM: LAPIC: Retry tune per-vCPU timer_advance_ns if
 adaptive tuning goes insane

On 05/07/19 17:23, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> Retry tune per-vCPU timer_advance_ns if adaptive tuning goes insane which 
> can happen sporadically in product environment.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> v1 -> v2:
>  * retry max 10 times if adaptive tuning goes insane

Is there any advantage at stopping the retry (also it should not be a
local variable of course).

Paolo

>  arch/x86/kvm/lapic.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 95affa5..bd0dbe5 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1538,6 +1538,7 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
>  	struct kvm_lapic *apic = vcpu->arch.apic;
>  	u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns;
>  	u64 ns;
> +	uint retry_count = 0;
>  
>  	/* too early */
>  	if (advance_expire_delta < 0) {
> @@ -1556,8 +1557,10 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
>  	if (abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_DONE)
>  		apic->lapic_timer.timer_advance_adjust_done = true;
>  	if (unlikely(timer_advance_ns > 5000)) {
> -		timer_advance_ns = 0;
> -		apic->lapic_timer.timer_advance_adjust_done = true;
> +		timer_advance_ns = 1000;
> +		apic->lapic_timer.timer_advance_adjust_done = false;
> +		if (++retry_count > 10)
> +			apic->lapic_timer.timer_advance_adjust_done = true;
>  	}
>  	apic->lapic_timer.timer_advance_ns = timer_advance_ns;
>  }
>