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Message-ID: <3a0c3397302d59ea313e079435a18bf1b9a43474.camel@redhat.com>
Subject: Re: [PATCH v2 16/43] KVM: Don't redo ktime_get() when calculating
 halt-polling stop/deadline
From:   Maxim Levitsky <mlevitsk@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Marc Zyngier <maz@kernel.org>,
        Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Anup Patel <anup.patel@wdc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>
Cc:     James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Atish Patra <atish.patra@wdc.com>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-mips@vger.kernel.org, kvm@vger.kernel.org,
        kvm-ppc@vger.kernel.org, kvm-riscv@lists.infradead.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        David Matlack <dmatlack@google.com>,
        Oliver Upton <oupton@google.com>,
        Jing Zhang <jingzhangos@google.com>
Date:   Wed, 27 Oct 2021 17:35:45 +0300
In-Reply-To: <0072221e-02e8-4d60-9b0f-80d8c423bf4e@redhat.com>
References: <20211009021236.4122790-1-seanjc@google.com>
         <20211009021236.4122790-17-seanjc@google.com>
         <0072221e-02e8-4d60-9b0f-80d8c423bf4e@redhat.com>
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On Mon, 2021-10-25 at 16:26 +0200, Paolo Bonzini wrote:
> On 09/10/21 04:12, 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.
> 
> Using "start" makes the change even more obvious, so:
> 
>      Calculate the halt-polling "stop" time using "start" instead of redoing
>      ktime_get().  In practice, the numbers involved are in the noise (e.g.,
>      in the happy case where hardware correctly predicts do_halt_poll and
>      there are no interrupts, "start" is probably only a few cycles old)
>      and either approach is perfectly ok.  But it's more precise to count
>      any extra latency toward the halt-polling time.
> 
> Paolo
> 
Agreed.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>