linux-kernel - Re: [PATCH v2 16/43] KVM: Don't redo ktime_get() when calculating halt-polling stop/deadline

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Message-ID: <3a0c3397302d59ea313e079435a18bf1b9a43474.camel@redhat.com>
Date:   Wed, 27 Oct 2021 17:35:45 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>
Cc:     James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atish.patra@....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, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        David Matlack <dmatlack@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH v2 16/43] KVM: Don't redo ktime_get() when calculating
 halt-polling stop/deadline

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@...hat.com>