linux-kernel - Re: [PATCH RFC v3 1/9] KVM: s390: optimize detection of started vcpus

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Date:   Tue, 22 Aug 2017 13:31:27 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-mips@...ux-mips.org, kvm-ppc@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Christoffer Dall <cdall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        James Hogan <james.hogan@...tec.com>,
        Paul Mackerras <paulus@...abs.org>,
        Alexander Graf <agraf@...e.com>
Subject: Re: [PATCH RFC v3 1/9] KVM: s390: optimize detection of started vcpus

On 21.08.2017 22:35, Radim Krčmář wrote:
> We can add a variable instead of scanning all online VCPUs to know how
> many are started.  We can't trivially tell which VCPU is the last one,
> though.

You could keep the started vcpus in a list. Then you might drop unsigned
started_vcpus;

No started vcpus: Start pointer NULL
Single started vcpu: Only one element in the list (easy to check)
> 1 started vcpus: More than one element int he list (easy to check)


> 
> Suggested-by: Cornelia Huck <cohuck@...hat.com>
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
> ---
>  arch/s390/include/asm/kvm_host.h |  1 +
>  arch/s390/kvm/kvm-s390.c         | 39 +++++++++++++++------------------------
>  2 files changed, 16 insertions(+), 24 deletions(-)
> 
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index a409d5991934..be0d0bdf585b 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -735,6 +735,7 @@ struct kvm_arch{
>  	struct mutex ipte_mutex;
>  	struct ratelimit_state sthyi_limit;
>  	spinlock_t start_stop_lock;
> +	unsigned started_vcpus;
>  	struct sie_page2 *sie_page2;
>  	struct kvm_s390_cpu_model model;
>  	struct kvm_s390_crypto crypto;
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 9f23a9e81a91..1534778a3c66 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -3414,25 +3414,17 @@ static void __enable_ibs_on_vcpu(struct kvm_vcpu *vcpu)
>  
>  void kvm_s390_vcpu_start(struct kvm_vcpu *vcpu)
>  {
> -	int i, online_vcpus, started_vcpus = 0;
> -
>  	if (!is_vcpu_stopped(vcpu))
>  		return;
>  
>  	trace_kvm_s390_vcpu_start_stop(vcpu->vcpu_id, 1);
>  	/* Only one cpu at a time may enter/leave the STOPPED state. */
>  	spin_lock(&vcpu->kvm->arch.start_stop_lock);
> -	online_vcpus = atomic_read(&vcpu->kvm->online_vcpus);
>  
> -	for (i = 0; i < online_vcpus; i++) {
> -		if (!is_vcpu_stopped(vcpu->kvm->vcpus[i]))
> -			started_vcpus++;
> -	}
> -
> -	if (started_vcpus == 0) {
> +	if (vcpu->kvm->arch.started_vcpus == 0) {
>  		/* we're the only active VCPU -> speed it up */
>  		__enable_ibs_on_vcpu(vcpu);
> -	} else if (started_vcpus == 1) {
> +	} else if (vcpu->kvm->arch.started_vcpus == 1) {
>  		/*
>  		 * As we are starting a second VCPU, we have to disable
>  		 * the IBS facility on all VCPUs to remove potentially
> @@ -3441,6 +3433,8 @@ void kvm_s390_vcpu_start(struct kvm_vcpu *vcpu)
>  		__disable_ibs_on_all_vcpus(vcpu->kvm);
>  	}
>  
> +	vcpu->kvm->arch.started_vcpus++;
> +
>  	atomic_andnot(CPUSTAT_STOPPED, &vcpu->arch.sie_block->cpuflags);
>  	/*
>  	 * Another VCPU might have used IBS while we were offline.
> @@ -3453,16 +3447,12 @@ void kvm_s390_vcpu_start(struct kvm_vcpu *vcpu)
>  
>  void kvm_s390_vcpu_stop(struct kvm_vcpu *vcpu)
>  {
> -	int i, online_vcpus, started_vcpus = 0;
> -	struct kvm_vcpu *started_vcpu = NULL;
> -
>  	if (is_vcpu_stopped(vcpu))
>  		return;
>  
>  	trace_kvm_s390_vcpu_start_stop(vcpu->vcpu_id, 0);
>  	/* Only one cpu at a time may enter/leave the STOPPED state. */
>  	spin_lock(&vcpu->kvm->arch.start_stop_lock);
> -	online_vcpus = atomic_read(&vcpu->kvm->online_vcpus);
>  
>  	/* SIGP STOP and SIGP STOP AND STORE STATUS has been fully processed */
>  	kvm_s390_clear_stop_irq(vcpu);
> @@ -3470,19 +3460,20 @@ void kvm_s390_vcpu_stop(struct kvm_vcpu *vcpu)
>  	atomic_or(CPUSTAT_STOPPED, &vcpu->arch.sie_block->cpuflags);
>  	__disable_ibs_on_vcpu(vcpu);
>  
> -	for (i = 0; i < online_vcpus; i++) {
> -		if (!is_vcpu_stopped(vcpu->kvm->vcpus[i])) {
> -			started_vcpus++;
> -			started_vcpu = vcpu->kvm->vcpus[i];
> -		}
> -	}
> +	vcpu->kvm->arch.started_vcpus--;
> +
> +	if (vcpu->kvm->arch.started_vcpus == 1) {
> +		struct kvm_vcpu *started_vcpu;
>  
> -	if (started_vcpus == 1) {
>  		/*
> -		 * As we only have one VCPU left, we want to enable the
> -		 * IBS facility for that VCPU to speed it up.
> +		 * As we only have one VCPU left, we want to enable the IBS
> +		 * facility for that VCPU to speed it up.
>  		 */
> -		__enable_ibs_on_vcpu(started_vcpu);
> +		kvm_for_each_vcpu(i, started_vcpu, vcpu->kvm)
> +			if (!is_vcpu_stopped(started_vcpu)) {
> +				__enable_ibs_on_vcpu(started_vcpu);
> +				break;
> +			}
>  	}
>  
>  	spin_unlock(&vcpu->kvm->arch.start_stop_lock);
> 


-- 

Thanks,

David