Re: [PATCH] ipv4: Early TCP socket demux.

[Eric Dumazet <eric.dumazet@gmail.com>]

On Mon, 2012-06-18 at 21:15 -0700, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@gmail.com>

> > This reminds the idea of having seperate dst per tcp socket, to remove
> > the dst refcnt contention as well.
> 
> I'm leery of this.
> 
> We're going to move towards having dst entries more strongly shared
> as we move to remove the routing cache.
> 
> In fact I have another short-term change planned that adjusts which
> keys the routing cache uses based upon what kinds of keys are actually
> active in the current FIB rule configuration.
> 
> I think we want to encourage sharing and make the route footprint
> smaller rather than expanding it's size artificually on even the
> socket level.
> 
> If you really care about this refcount problem, then it's another
> reason to never orphan socket sourced packets.  Then we wouldn't need
> to ever refcount the route just to send a packet, we'd just use the
> implicit reference held by the socket instead.  Socket route releasing
> would be held back by the presence of any packets in the socket send
> queue.  If we have to reset the dst mis-lifetime due to route flushes,
> we'd need to use a specific packet as a sequence point.
> 
> That to me sounds like a more reasonable approach than just making
> more and more routes.

We already don't touch dst refcnt on TCP xmit path, unless packet is
parked in Qdisc queue...

But with BQL this is becoming less effective.