Currently the MTU of vlan netdevice is set to the same MTU of the lower device, which requires the underlying device to handle it as the comment has indicated:
/* need 4 bytes for extra VLAN header info, * hope the underlying device can handle it. */ new_dev->mtu = real_dev->mtu; Currently most of the physical netdevs seems to handle above by reversing 2 * VLAN_HLEN for L2 packet len. But for vlan netdev over vxlan netdev case, the vxlan does not seems to reverse the vlan header for vlan device, which may cause performance degradation because vxlan may emit a packet that exceed the MTU of the physical netdev, and cause the software TSO to happen in ip_finish_output_gso(), software TSO call stack as below: => ftrace_graph_call => tcp_gso_segment => tcp4_gso_segment => inet_gso_segment => skb_mac_gso_segment => skb_udp_tunnel_segment => udp4_ufo_fragment => inet_gso_segment => skb_mac_gso_segment => __skb_gso_segment => __ip_finish_output => ip_output => ip_local_out => iptunnel_xmit => udp_tunnel_xmit_skb => vxlan_xmit_one => vxlan_xmit => dev_hard_start_xmit => __dev_queue_xmit => dev_queue_xmit => vlan_dev_hard_start_xmit => dev_hard_start_xmit => __dev_queue_xmit => dev_queue_xmit => neigh_resolve_output => ip_finish_output2 => __ip_finish_output => ip_output => ip_local_out => __ip_queue_xmit => ip_queue_xmit => __tcp_transmit_skb => tcp_write_xmit => __tcp_push_pending_frames => tcp_push => tcp_sendmsg_locked => tcp_sendmsg => inet_sendmsg => sock_sendmsg => sock_write_iter => new_sync_write => __vfs_write => vfs_write => ksys_write => __arm64_sys_write => el0_svc_common.constprop.0 => el0_svc_handler => el0_svc This patch set initial MTU of the vlan device to the MTU of the lower device minus vlan header to handle the above case. Signed-off-by: Yunsheng Lin <linyunsh...@huawei.com> --- net/8021q/vlan.c | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c index 54728d2..0c26b92 100644 --- a/net/8021q/vlan.c +++ b/net/8021q/vlan.c @@ -261,10 +261,9 @@ static int register_vlan_device(struct net_device *real_dev, u16 vlan_id) return -ENOBUFS; dev_net_set(new_dev, net); - /* need 4 bytes for extra VLAN header info, - * hope the underlying device can handle it. - */ - new_dev->mtu = real_dev->mtu; + new_dev->mtu = real_dev->mtu - VLAN_HLEN; + if (new_dev->mtu < ETH_MIN_MTU) + new_dev->mtu = ETH_MIN_MTU; vlan = vlan_dev_priv(new_dev); vlan->vlan_proto = htons(ETH_P_8021Q); -- 2.8.1