From: Eric Dumazet <redacted>
Date: Mon, 24 Nov 2008 11:14:29 +0100

So maybe we could make ip_append_data() (or its callers) a
litle bit smarter, avoiding increment/decrement if possible.

These ideas are interesting but hard to make work.

I think the receive path has more chance of getting gains
from this, to be honest.

One third (effectively) of TCP stream packets are ACKs and
freed immediately.  This means that the looked up route does
not escape the packet receive path.  So we could elide the
counter increment in that case.

In fact, once we queue even TCP data, there is no need for
that cached skb->dst route any longer.

So pretty much all TCP packets could avoid the dst refcounting
on receive.