On Mon, 21 Oct 2019 20:26:03 +0800 Yunsheng Lin <linyunsh...@huawei.com> wrote:
> 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> The MTU is visible to user space in many tools, and Linux (and BSD) have always treated VLAN header as not part of the MTU. You can't change that now.
