linux-kernel - Re: [PATCH] KVM: LAPIC: Don't kick vCPU which is injecting already-expired timer

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Date:   Fri, 21 Aug 2020 21:01:14 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: LAPIC: Don't kick vCPU which is injecting
 already-expired timer

On Fri, Aug 21, 2020 at 05:17:34PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> The kick after setting KVM_REQ_PENDING_TIMER is used to handle the timer 
> fires on a different pCPU which vCPU is running on, we don't need this 
> kick when injecting already-expired timer, this kick is expensive since 
> memory barrier, rcu, preemption disable/enable operations. This patch 
> reduces the overhead by don't kick vCPU which is injecting already-expired 
> timer.

This should also call out the VMX preemption timer case, which also passes
from_timer_fn=false but doesn't need a kick because kvm_lapic_expired_hv_timer()
is called from the target vCPU.
 
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/lapic.c | 2 +-
>  arch/x86/kvm/x86.c   | 5 +++--
>  arch/x86/kvm/x86.h   | 2 +-
>  3 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 248095a..5b5ae66 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1642,7 +1642,7 @@ static void apic_timer_expired(struct kvm_lapic *apic, bool from_timer_fn)
>  	}
>  
>  	atomic_inc(&apic->lapic_timer.pending);
> -	kvm_set_pending_timer(vcpu);
> +	kvm_set_pending_timer(vcpu, from_timer_fn);

My vote would be to open code kvm_set_pending_timer() here and drop the
helper, i.e.

	kvm_make_request(KVM_REQ_PENDING_TIMER, vcpu);
	if (from_timer_fn)
		kvm_vcpu_kick(vcpu);

with that and an updated changelog:

Reviewed-by: Sean Christopherson <sean.j.christopherson@...el.com>

>  }
>  
>  static void start_sw_tscdeadline(struct kvm_lapic *apic)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 599d732..2a45405 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1778,10 +1778,11 @@ static s64 get_kvmclock_base_ns(void)
>  }
>  #endif
>  
> -void kvm_set_pending_timer(struct kvm_vcpu *vcpu)
> +void kvm_set_pending_timer(struct kvm_vcpu *vcpu, bool should_kick)
>  {
>  	kvm_make_request(KVM_REQ_PENDING_TIMER, vcpu);
> -	kvm_vcpu_kick(vcpu);
> +	if (should_kick)
> +		kvm_vcpu_kick(vcpu);
>  }
>  
>  static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock)
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 995ab69..0eaae9c 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -246,7 +246,7 @@ static inline bool kvm_vcpu_latch_init(struct kvm_vcpu *vcpu)
>  	return is_smm(vcpu) || kvm_x86_ops.apic_init_signal_blocked(vcpu);
>  }
>  
> -void kvm_set_pending_timer(struct kvm_vcpu *vcpu);
> +void kvm_set_pending_timer(struct kvm_vcpu *vcpu, bool should_kick);
>  void kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip);
>  
>  void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr);
> -- 
> 2.7.4
>