linux-kernel - Re: [PATCH 1/5] KVM: simplify branch check in host poll code

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Message-ID: <20191101210331.GA20061@amt.cnet>
Date:   Fri, 1 Nov 2019 19:03:32 -0200
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Zhenzhong Duan <zhenzhong.duan@...cle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        joao.m.martins@...cle.com, rafael.j.wysocki@...el.com,
        rkrcmar@...hat.com, pbonzini@...hat.com
Subject: Re: [PATCH 1/5] KVM: simplify branch check in host poll code

On Sat, Oct 26, 2019 at 11:23:55AM +0800, Zhenzhong Duan wrote:
> Remove redundant check.
> 
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@...cle.com>
> ---
>  virt/kvm/kvm_main.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> 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)
>  		} else if (halt_poll_ns) {
>  			if (block_ns <= vcpu->halt_poll_ns)
>  				;
> -			/* we had a long block, shrink polling */
> -			else if (vcpu->halt_poll_ns && block_ns > halt_poll_ns)
> -				shrink_halt_poll_ns(vcpu);
>  			/* we had a short halt and our poll time is too small */
> -			else 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:

	1) Multiplication
	2) Cap that back to halt_poll_ns
	3) Invoke the trace_kvm_halt_poll_ns_grow tracepoint
	   (when in fact vcpu->halt_poll_ns did not grow).