Subject: Re: [PATCH 03/14] KVM: Refactor and document halt-polling stats update helper

From: David Matlack <[email-masked]>
Date: Tue, 28 Sep 2021 19:01:38 +0000

On Fri, Sep 24, 2021 at 05:55:17PM -0700, Sean Christopherson wrote:
> Add a comment to document that halt-polling is considered successful even
> if the polling loop itself didn't detect a wake event, i.e. if a wake
> event was detect in the final kvm_vcpu_check_block().  Invert the param
> to the update helper so that the helper is a dumb function that is "told"
> whether or not polling was successful, as opposed to having it determinine
> success/failure based on blocking behavior.
> 
> Opportunistically tweak the params to the update helper to reduce the
> line length for the call site so that it fits on a single line, and so
> that the prototype conforms to the more traditional kernel style.
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <[email-masked]>

Reviewed-by: David Matlack <[email-masked]>

> ---
>  virt/kvm/kvm_main.c | 20 +++++++++++++-------
>  1 file changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 8b33f5045b4d..12fe91a0a4c8 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3199,13 +3199,15 @@ static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
>  	return ret;
>  }
>  
> -static inline void
> -update_halt_poll_stats(struct kvm_vcpu *vcpu, u64 poll_ns, bool waited)
> +static inline void update_halt_poll_stats(struct kvm_vcpu *vcpu, ktime_t start,
> +					  ktime_t end, bool success)
>  {
> -	if (waited)
> -		vcpu->stat.generic.halt_poll_fail_ns += poll_ns;
> -	else
> +	u64 poll_ns = ktime_to_ns(ktime_sub(end, start));
> +
> +	if (success)
>  		vcpu->stat.generic.halt_poll_success_ns += poll_ns;
> +	else
> +		vcpu->stat.generic.halt_poll_fail_ns += poll_ns;
>  }
>  
>  /*
> @@ -3274,9 +3276,13 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>  	kvm_arch_vcpu_unblocking(vcpu);
>  	block_ns = ktime_to_ns(cur) - ktime_to_ns(start);
>  
> +	/*
> +	 * Note, halt-polling is considered successful so long as the vCPU was
> +	 * never actually scheduled out, i.e. even if the wake event arrived
> +	 * after of the halt-polling loop itself, but before the full wait.
> +	 */
>  	if (do_halt_poll)
> -		update_halt_poll_stats(
> -			vcpu, ktime_to_ns(ktime_sub(poll_end, start)), waited);
> +		update_halt_poll_stats(vcpu, start, poll_end, !waited);
>  
>  	if (halt_poll_allowed) {
>  		if (!vcpu_valid_wakeup(vcpu)) {
> -- 
> 2.33.0.685.g46640cef36-goog
> 

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