Re: [PATCH net] vxlan: update skb dst pmtu on tx path

On Wed, Dec 20, 2017 at 12:12 AM, David Miller [off-list ref] wrote:

From: Xin Long <lucien.xin@gmail.com>
Date: Mon, 18 Dec 2017 14:20:56 +0800

quoted

Unlike ip tunnels, now vxlan doesn't do any pmtu update for
upper dst pmtu, even if it doesn't match the lower dst pmtu
any more.

The problem can be reproduced when reducing the vxlan lower
dev's pmtu when running netperf. In jianlin's testing, the
performance went to 1/7 of the previous.

This patch is to update the upper dst pmtu to match the lower
dst pmtu on tx path so that packets can be sent out even when
lower dev's pmtu has been changed.

It also works for metadata dst.

Note that this patch doesn't process any pmtu icmp packet.
But even in the future, the support for pmtu icmp packets
process of udp tunnels will also needs this.

The same thing will be done for geneve in another patch.

Signed-off-by: Xin Long <lucien.xin@gmail.com>

Yikes...

You're going to have to find a way to fix this without
invoking ->update_pmtu() on every single transmit.  That's
really excessive, especially for an operation which is
going to be a NOP %99.9999 of the time.

understand, I couldn't find a better way,  and all iptunnels are
doing it in this way.

Or is it possible to go with an unlikely here ?

                if (unlikely(skb_dst(skb) && mtu < dst_mtu(skb_dst(skb))))
                        skb_dst(skb)->ops->update_pmtu(skb_dst(skb), NULL,
                                                       skb, mtu);

We need some way, instead, for the MTU change event to propagate
properly.  I know this might be hard, but doing this in the transmit
handler on every packet to deal with it is not the way to go.

how about doing it in vxlan_get_route():

@@ -1896,6 +1896,13 @@ static struct rtable *vxlan_get_route(struct

vxlan_dev *vxlan, struct net_device
                *saddr = fl4.saddr;
                if (use_cache)
                        dst_cache_set_ip4(dst_cache, &rt->dst, fl4.saddr);
+
+               if (skb_dst(skb)) {
+                       int mtu = dst_mtu(ndst) - VXLAN_HEADROOM;
+
+                       skb_dst(skb)->ops->update_pmtu(skb_dst(skb), NULL,
+                                                      skb, mtu);
+               }


This would do it only when no dst_cache and it has to do real route lookup.

Note that even when update_pmtu is hit, mostly it will do nothing and
just return
as usually new mtu >= skb_dst(skb)'s pmtu.

Thanks.