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Subject: Re: [PATCH] KVM: Boost vCPU candidiate in user mode which is
 delivering interrupt
To:     Wanpeng Li <kernellwp@gmail.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
References: <1618542490-14756-1-git-send-email-wanpengli@tencent.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <9c49c6ff-d896-e6a5-c051-b6707f6ec58a@redhat.com>
Date:   Sat, 17 Apr 2021 15:09:08 +0200
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On 16/04/21 05:08, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> Both lock holder vCPU and IPI receiver that has halted are condidate for
> boost. However, the PLE handler was originally designed to deal with the
> lock holder preemption problem. The Intel PLE occurs when the spinlock
> waiter is in kernel mode. This assumption doesn't hold for IPI receiver,
> they can be in either kernel or user mode. the vCPU candidate in user mode
> will not be boosted even if they should respond to IPIs. Some benchmarks
> like pbzip2, swaptions etc do the TLB shootdown in kernel mode and most
> of the time they are running in user mode. It can lead to a large number
> of continuous PLE events because the IPI sender causes PLE events
> repeatedly until the receiver is scheduled while the receiver is not
> candidate for a boost.
> 
> This patch boosts the vCPU candidiate in user mode which is delivery
> interrupt. We can observe the speed of pbzip2 improves 10% in 96 vCPUs
> VM in over-subscribe scenario (The host machine is 2 socket, 48 cores,
> 96 HTs Intel CLX box). There is no performance regression for other
> benchmarks like Unixbench spawn (most of the time contend read/write
> lock in kernel mode), ebizzy (most of the time contend read/write sem
> and TLB shoodtdown in kernel mode).
>   
> +bool kvm_arch_interrupt_delivery(struct kvm_vcpu *vcpu)
> +{
> +	if (vcpu->arch.apicv_active && static_call(kvm_x86_dy_apicv_has_pending_interrupt)(vcpu))
> +		return true;
> +
> +	return false;
> +}

Can you reuse vcpu_dy_runnable instead of this new function?

Paolo


>   bool kvm_arch_vcpu_in_kernel(struct kvm_vcpu *vcpu)
>   {
>   	return vcpu->arch.preempted_in_kernel;
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 3b06d12..5012fc4 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -954,6 +954,7 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu);
>   bool kvm_arch_vcpu_in_kernel(struct kvm_vcpu *vcpu);
>   int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu);
>   bool kvm_arch_dy_runnable(struct kvm_vcpu *vcpu);
> +bool kvm_arch_interrupt_delivery(struct kvm_vcpu *vcpu);
>   int kvm_arch_post_init_vm(struct kvm *kvm);
>   void kvm_arch_pre_destroy_vm(struct kvm *kvm);
>   
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 0a481e7..781d2db 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3012,6 +3012,11 @@ static bool vcpu_dy_runnable(struct kvm_vcpu *vcpu)
>   	return false;
>   }
>   
> +bool __weak kvm_arch_interrupt_delivery(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
>   void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
>   {
>   	struct kvm *kvm = me->kvm;
> @@ -3045,6 +3050,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
>   			    !vcpu_dy_runnable(vcpu))
>   				continue;
>   			if (READ_ONCE(vcpu->preempted) && yield_to_kernel_mode &&
> +				!kvm_arch_interrupt_delivery(vcpu) &&
>   				!kvm_arch_vcpu_in_kernel(vcpu))
>   				continue;
>   			if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
>