netdev - Re: [PATCH net-next 5/5] veth: implement ndo_set_rx

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Date:	Tue, 23 Feb 2016 11:21:26 -0800
From:	pravin shelar <pshelar@....org>
To:	Paolo Abeni <pabeni@...hat.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Pravin Shelar <pshelar@...ira.com>,
	Jesse Gross <jesse@...nel.org>,
	Flavio Leitner <fbl@...close.org>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [PATCH net-next 5/5] veth: implement ndo_set_rx_headroom

On Tue, Feb 23, 2016 at 4:53 AM, Paolo Abeni <pabeni@...hat.com> wrote:
> The rx headroom for veth dev is the peer device needed_headroom.
> Avoid ping-pong updates setting the private flag IFF_PHONY_HEADROOM.
>
> This avoids skb head reallocation when forwarding from a veth dev
> towards a device adding some kind of encapsulation.
>
> When forwarding frames below the MTU size towards a vxlan device,
> this gives about 10% performance speed-up when OVS is used to connect
> the veth and the vxlan device and a little more when using a
> plain Linux bridge.
>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
>  drivers/net/veth.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
>
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index ba21d07..4f30a6a 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -35,6 +35,7 @@ struct pcpu_vstats {
>  struct veth_priv {
>         struct net_device __rcu *peer;
>         atomic64_t              dropped;
> +       unsigned                requested_headroom;
>  };
>
>  /*
> @@ -271,6 +272,29 @@ static int veth_get_iflink(const struct net_device *dev)
>         return iflink;
>  }
>
> +static void veth_set_rx_headroom(struct net_device *dev, int new_hr)
> +{
> +       struct veth_priv *peer_priv, *priv = netdev_priv(dev);
> +       struct net_device *peer;
> +
> +       if (new_hr < 0)
> +               new_hr = 0;
> +
> +       rcu_read_lock();
> +       peer = rcu_dereference(priv->peer);
> +       if (unlikely(!peer))
> +               goto out;
> +
> +       peer_priv = netdev_priv(peer);
> +       priv->requested_headroom = new_hr;
> +       new_hr = max(priv->requested_headroom, peer_priv->requested_headroom);
> +       dev->needed_headroom = new_hr;
> +       peer->needed_headroom = new_hr;
> +
> +out:
> +       rcu_read_unlock();
> +}
> +
I am not sure why new priv->requested_headroom is introduced. I think
you can just compare new_hr with dev->needed_headroom and set the max
value to both devices.