Re: [RFC PATCH net-next 03/12] vhost_net: introduce vhost_has_more_pkts()

Jesse Brandeburg
2018-05-21T16:39:12Z

On Mon, 21 May 2018 17:04:24 +0800 Jason wrote:
> Signed-off-by: Jason Wang <jasowang@redhat.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;
> }