linux-kernel - Re: [RFC PATCH net-next 03/12] vhost_net: introduce vhost_has_more

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Message-ID: <20180521093908.00006747@intel.com>
Date:   Mon, 21 May 2018 09:39:08 -0700
From:   Jesse Brandeburg <jesse.brandeburg@...el.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     <mst@...hat.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>,
        jesse.brandeburg@...el.com
Subject: Re: [RFC PATCH net-next 03/12] vhost_net: introduce
 vhost_has_more_pkts()

On Mon, 21 May 2018 17:04:24 +0800 Jason wrote:
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>  drivers/vhost/net.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index de544ee..4ebac76 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -485,6 +485,13 @@ static bool vhost_exceeds_weight(int pkts, int total_len)
>  	       unlikely(pkts >= VHOST_NET_PKT_WEIGHT);
>  }
>  
> +static bool vhost_has_more_pkts(struct vhost_net *net,
> +				struct vhost_virtqueue *vq)
> +{
> +	return !vhost_vq_avail_empty(&net->dev, vq) &&
> +	       likely(!vhost_exceeds_maxpend(net));

This really seems like mis-use of likely/unlikely, in the middle of a
sequence of operations that will always be run when this function is
called.  I think you should remove the likely from this helper,
especially, and control the branch from the branch point.


> +}
> +
>  /* Expects to be always run from workqueue - which acts as
>   * read-size critical section for our kind of RCU. */
>  static void handle_tx(struct vhost_net *net)
> @@ -578,8 +585,7 @@ static void handle_tx(struct vhost_net *net)
>  		}
>  		total_len += len;
>  		if (total_len < VHOST_NET_WEIGHT &&
> -		    !vhost_vq_avail_empty(&net->dev, vq) &&
> -		    likely(!vhost_exceeds_maxpend(net))) {
> +		    vhost_has_more_pkts(net, vq)) {

Yes, I know it came from here, but likely/unlikely are for branch
control, so they should encapsulate everything inside the if, unless
I'm mistaken.

>  			msg.msg_flags |= MSG_MORE;
>  		} else {
>  			msg.msg_flags &= ~MSG_MORE;
> @@ -605,7 +611,7 @@ static void handle_tx(struct vhost_net *net)
>  		else
>  			vhost_zerocopy_signal_used(net, vq);
>  		vhost_net_tx_packet(net);
> -		if (unlikely(vhost_exceeds_weight(++sent_pkts, total_len))) {
> +		if (vhost_exceeds_weight(++sent_pkts, total_len)) {

You should have kept the unlikely here, and not had it inside the
helper (as per the previous patch.  Also, why wasn't this change part
of the previous patch?

>  			vhost_poll_queue(&vq->poll);
>  			break;
>  		}