Re: [PATCH V3 2/2] vhost_net: conditionally enable tx polling

Michael S. Tsirkin
2016-06-07T12:26:31Z

On Wed, Jun 01, 2016 at 01:56:34AM -0400, Jason Wang wrote:
> We always poll tx for socket, this is sub optimal since:
>
> - it will be only used when we exceed the sndbuf of the socket.
> - since we use two independent polls for tx and vq, this will slightly
> increase the waitqueue traversing time and more important, vhost
> could not benefit from commit
> 9e641bdcfa4ef4d6e2fbaa59c1be0ad5d1551fd5 ("net-tun: restructure
> tun_do_read for better sleep/wakeup efficiency") even if we've
> stopped rx polling during handle_rx since tx poll were still left in
> the waitqueue.
>
> Fix this by conditionally enable tx polling only when -EAGAIN were
> met.
>
> Test shows about 8% improvement on guest rx pps.
>
> Before: ~1350000
> After: ~1460000
>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
> drivers/vhost/net.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 1d3e45f..e75ffcc 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -378,6 +378,7 @@ static void handle_tx(struct vhost_net *net)
> goto out;
>
> vhost_disable_notify(&net->dev, vq);
> + vhost_net_disable_vq(net, vq);
>
> hdr_size = nvq->vhost_hlen;
> zcopy = nvq->ubufs;
> @@ -459,6 +460,8 @@ static void handle_tx(struct vhost_net *net)
> % UIO_MAXIOV;
> }
> vhost_discard_vq_desc(vq, 1);
> + if (err == -EAGAIN)
> + vhost_net_enable_vq(net, vq);
> break;
> }
> if (err != len)

This seems rather risky. What if TX failed for some other reason?
Polling won't ever be re-enabled ...

> --
> 1.8.3.1