linux-kernel - Re: [PATCH 12/14] KVM: Don't redo ktime_get() when calculating halt-polling stop/deadline

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Date:   Tue, 28 Sep 2021 22:08:33 +0000
From:   David Matlack <dmatlack@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH 12/14] KVM: Don't redo ktime_get() when calculating
 halt-polling stop/deadline

On Fri, Sep 24, 2021 at 05:55:26PM -0700, Sean Christopherson wrote:
> Calculate the halt-polling "stop" time using "cur" instead of redoing
> ktime_get().  In the happy case where hardware correctly predicts
> do_halt_poll, "cur" is only a few cycles old.  And if the branch is
> mispredicted, arguably that extra latency should count toward the
> halt-polling time.
> 
> In all likelihood, the numbers involved are in the noise and either
> approach is perfectly ok.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: David Matlack <dmatlack@...gle.com>

> ---
>  virt/kvm/kvm_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 2980d2b88559..80f78daa6b8d 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3267,7 +3267,7 @@ void kvm_vcpu_halt(struct kvm_vcpu *vcpu)
>  
>  	start = cur = poll_end = ktime_get();
>  	if (do_halt_poll) {
> -		ktime_t stop = ktime_add_ns(ktime_get(), vcpu->halt_poll_ns);
> +		ktime_t stop = ktime_add_ns(cur, vcpu->halt_poll_ns);
>  
>  		do {
>  			/*
> -- 
> 2.33.0.685.g46640cef36-goog
>