netdev - Re: [PATCH net-next 1/2] udp: UDP Fast Port

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Date:	Tue, 29 Jul 2014 15:29:37 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	therbert@...gle.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 1/2] udp: UDP Fast Port

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 28 Jul 2014 16:20:22 +0200

> On Sat, 2014-07-26 at 15:29 -0700, Tom Herbert wrote:
>> This patch implements fast ports in UDP. The idea is that a kernel
>> module registers a port and associated receive function. In UDP receive,
>> the list of fast ports is scanned and the destinaton port is matched
>> against the registered port. If there is a match, then the receive
>> function is called to process the packet. When a UDP fast port is used,
>> we can receive encap'ed packets without performing or accessing a
>> socket. This is a performance gain, especially since we don't need to
>> take socket ref count on every packet.
>> 
>> Signed-off-by: Tom Herbert <therbert@...gle.com>
>> ---
>>  include/net/udp.h | 23 +++++++++++++++++++
>>  net/ipv4/udp.c    | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
>>  net/ipv6/udp.c    | 21 +++++++++++++++--
>>  3 files changed, 108 insertions(+), 5 deletions(-)
> 
> I find this very hacky. I do not think we should have different sorts of
> UDP sockets.
> 
> If we believe we need a fast UDP path, without socket refcount being
> touched, we should remove the SLAB_DESTROY_BY_RCU and switch to rcu
> socket freeing (call_rcu())
> 
> At the time we added RCU lookups to UDP, we hadn't vxlan, and fear was
> that the extra RCU grace period at socket dismantle was too heavy cost,
> like TCP sockets.
> 
> Meanwhile, RCU cleanups got lot of attention, and UDP sockets serve as
> tunnels vehicle.

I remember trying to move socket destruction fully to call_rcu() and
noticing real increases in socket create/destroy latency.

But like Eric I dislike these special cased demux objects, just do
something with real sockets, UDP demux is already too complex.

One idea is to add a ->decap() operation to the proto ops, and if
non-NULL the socket demux invokes the ->decap() while still in the RCU
protected section and returns a NULL socket.  It would elide the
refcount in this path as well.
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