netdev - Re: [net-next 3/3] ip_gre: cache the device mtu hard_header

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Date:   Wed, 11 Oct 2017 17:54:52 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     William Tu <u9012063@...il.com>
Cc:     network dev <netdev@...r.kernel.org>,
        David Laight <David.Laight@...lab.com>,
        davem <davem@...emloft.net>
Subject: Re: [net-next 3/3] ip_gre: cache the device mtu hard_header_len calc

On Tue, Oct 10, 2017 at 4:47 AM, William Tu <u9012063@...il.com> wrote:
> The patch introduces ip_tunnel->ether_mtu fields to cache the value of
> dev->mtu + dev->hard_header_len.  This avoids the arithmetic operation
> on every packet.
>
> Signed-off-by: William Tu <u9012063@...il.com>
> Cc: David Laight <David.Laight@...lab.com>
> ---
>  include/net/ip_tunnels.h | 1 +
>  net/ipv4/ip_gre.c        | 8 ++++----
>  net/ipv4/ip_tunnel.c     | 3 +++
>  3 files changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/include/net/ip_tunnels.h b/include/net/ip_tunnels.h
> index b41a1e057fce..19565be26e13 100644
> --- a/include/net/ip_tunnels.h
> +++ b/include/net/ip_tunnels.h
> @@ -117,6 +117,7 @@ struct ip_tunnel {
>
>         /* This field used only by ERSPAN */
>         u32             index;          /* ERSPAN type II index */
> +       unsigned int    ether_mtu;      /* The mtu including the ether hdr */
>
>         struct dst_cache dst_cache;
ip_tunnel is a very common structure for various tunnels.
Adding ether_mtu into it to avoid  ONLY an addition operation ONLY for
erspan, I think it's not worth it.

>
> diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
> index 6e6e4c4811cc..994b8ddea0b1 100644
> --- a/net/ipv4/ip_gre.c
> +++ b/net/ipv4/ip_gre.c
> @@ -578,8 +578,8 @@ static void erspan_fb_xmit(struct sk_buff *skb, struct net_device *dev,
>         if (gre_handle_offloads(skb, false))
>                 goto err_free_rt;
>
> -       if (skb->len > dev->mtu + dev->hard_header_len) {
> -               pskb_trim(skb, dev->mtu + dev->hard_header_len);
> +       if (skb->len > tunnel->ether_mtu) {
> +               pskb_trim(skb, tunnel->ether_mtu);
>                 truncate = true;
>         }
>
> @@ -730,8 +730,8 @@ static netdev_tx_t erspan_xmit(struct sk_buff *skb,
>         if (skb_cow_head(skb, dev->needed_headroom))
>                 goto free_skb;
>
> -       if (skb->len > dev->mtu + dev->hard_header_len) {
> -               pskb_trim(skb, dev->mtu + dev->hard_header_len);
> +       if (skb->len > tunnel->ether_mtu) {
> +               pskb_trim(skb, tunnel->ether_mtu);
>                 truncate = true;
>         }
>
> diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
> index fe6fee728ce4..859af5b86802 100644
> --- a/net/ipv4/ip_tunnel.c
> +++ b/net/ipv4/ip_tunnel.c
> @@ -348,6 +348,7 @@ static int ip_tunnel_bind_dev(struct net_device *dev)
>
>         dev->needed_headroom = t_hlen + hlen;
>         mtu -= (dev->hard_header_len + t_hlen);
> +       tunnel->ether_mtu = mtu + dev->hard_header_len;
>
>         if (mtu < 68)
>                 mtu = 68;
> @@ -952,6 +953,7 @@ int __ip_tunnel_change_mtu(struct net_device *dev, int new_mtu, bool strict)
>         }
>
>         dev->mtu = new_mtu;
> +       tunnel->ether_mtu = new_mtu + dev->hard_header_len;
>         return 0;
>  }
>  EXPORT_SYMBOL_GPL(__ip_tunnel_change_mtu);
> @@ -1183,6 +1185,7 @@ int ip_tunnel_init(struct net_device *dev)
>
>         tunnel->dev = dev;
>         tunnel->net = dev_net(dev);
> +       tunnel->ether_mtu = dev->mtu + dev->hard_header_len;
>         strcpy(tunnel->parms.name, dev->name);
>         iph->version            = 4;
>         iph->ihl                = 5;
> --
> 2.7.4
>