linux-kernel - Re: [PATCH] KVM: Don't shrink/grow vCPU halt_poll

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Message-ID: <20190927144234.GD24889@linux.intel.com>
Date:   Fri, 27 Sep 2019 07:42:34 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH] KVM: Don't shrink/grow vCPU halt_poll_ns if host side
 polling is disabled

On Fri, Sep 27, 2019 at 04:27:02PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> Don't waste cycles to shrink/grow vCPU halt_poll_ns if host 
> side polling is disabled.
> 
> Cc: Marcelo Tosatti <mtosatti@...hat.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  virt/kvm/kvm_main.c | 28 +++++++++++++++-------------
>  1 file changed, 15 insertions(+), 13 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index e6de315..b368be4 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2359,20 +2359,22 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>  	kvm_arch_vcpu_unblocking(vcpu);
>  	block_ns = ktime_to_ns(cur) - ktime_to_ns(start);
>  
> -	if (!vcpu_valid_wakeup(vcpu))
> -		shrink_halt_poll_ns(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)
> +	if (!kvm_arch_no_poll(vcpu)) {

Can vcpu->halt_poll_ns be cached and used both here and in the similar
check above?  E.g.:

	unsigned int vcpu_halt_poll_ns;

	vcpu_halt_poll_ns = kvm_arch_no_poll(vcpu) ? 0 : vcpu->halt_poll_ns;

	if (vcpu_halt_poll_ns) {
		...
	}

> +		if (!vcpu_valid_wakeup(vcpu))
>  			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 &&
> -			block_ns < halt_poll_ns)
> -			grow_halt_poll_ns(vcpu);
> -	} else
> -		vcpu->halt_poll_ns = 0;
> +		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 &&
> +				block_ns < halt_poll_ns)
> +				grow_halt_poll_ns(vcpu);
> +		} else
> +			vcpu->halt_poll_ns = 0;


Not your code, but it'd be a good time to add braces to the 'if' and
'else'.  Per Documentation/process/coding-style.rst:

  Do not unnecessarily use braces where a single statement will do.

  ...

  This does not apply if only one branch of a conditional statement is a single
  statement; in the latter case use braces in both branches:

        if (condition) {
                do_this();
                do_that();
        } else {
                otherwise();
        }


> +	}
>  
>  	trace_kvm_vcpu_wakeup(block_ns, waited, vcpu_valid_wakeup(vcpu));
>  	kvm_arch_vcpu_block_finish(vcpu);
> -- 
> 2.7.4
>