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Date:   Fri, 1 Nov 2019 19:03:32 -0200
From:   Marcelo Tosatti <mtosatti@redhat.com>
To:     Zhenzhong Duan <zhenzhong.duan@oracle.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        joao.m.martins@oracle.com, rafael.j.wysocki@intel.com,
        rkrcmar@redhat.com, pbonzini@redhat.com
Subject: Re: [PATCH 1/5] KVM: simplify branch check in host poll code
Message-ID: <20191101210331.GA20061@amt.cnet>
References: <1572060239-17401-1-git-send-email-zhenzhong.duan@oracle.com>
 <1572060239-17401-2-git-send-email-zhenzhong.duan@oracle.com>
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On Sat, Oct 26, 2019 at 11:23:55AM +0800, Zhenzhong Duan wrote:
> Remove redundant check.
>=20
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
> ---
>  virt/kvm/kvm_main.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
>=20
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 67ef3f2..2ca2979 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2366,13 +2366,12 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>  =09=09} else if (halt_poll_ns) {
>  =09=09=09if (block_ns <=3D vcpu->halt_poll_ns)
>  =09=09=09=09;
> -=09=09=09/* we had a long block, shrink polling */
> -=09=09=09else if (vcpu->halt_poll_ns && block_ns > halt_poll_ns)
> -=09=09=09=09shrink_halt_poll_ns(vcpu);
>  =09=09=09/* we had a short halt and our poll time is too small */
> -=09=09=09else if (vcpu->halt_poll_ns < halt_poll_ns &&

This is not a redundant check: it avoids from calling
into grow_halt_poll_ns, which will do:

=091) Multiplication
=092) Cap that back to halt_poll_ns
=093) Invoke the trace_kvm_halt_poll_ns_grow tracepoint
=09   (when in fact vcpu->halt_poll_ns did not grow).