netdev - Re: [net-next 2/3] udp: Add udp early demux

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Date:	Wed, 2 Oct 2013 12:34:22 -0500
From:	Shawn Bohrer <sbohrer@...advisors.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, tomk@...advisors.com,
	netdev <netdev@...r.kernel.org>
Subject: Re: [net-next 2/3] udp: Add udp early demux

On Tue, Oct 01, 2013 at 01:52:49PM -0700, Eric Dumazet wrote:
> On Tue, 2013-10-01 at 14:33 -0500, Shawn Bohrer wrote:
> > The removal of the routing cache introduced a performance regression for
> > some UDP workloads since a dst lookup must be done for each packet.
> > This change caches the dst per socket in a similar manner to what we do
> > for TCP by implementing early_demux.
> > 
> > For UDP multicast we can only cache the dst if there is only one
> > receiving socket on the host.  Since caching only works when there is
> > one receiving socket we do the multicast socket lookup using RCU.
> 
> For unicast, we should find a matching socket for early demux only if
> this is a connected socket.
> 
> Otherwise, forwarding setups will break.
> 
> You probably need to add a minimum score to __udp4_lib_lookup()

Perhaps I'm missing something but I don't think a minimum score would
work because compute_score() and compute_score2() have several ways of
returning a score of lets say 4 and I don't think they all mean the
socket is connected.  Why not just test the socket returned by
__udp4_lib_lookup() to see if it is connected in
udp_v4_early_demux()?  Something like:

        sk = __udp4_lib_lookup(net, iph->saddr, uh->source,
                               iph->daddr, uh->dest, dif,
                               &udp_table);
        /* Only demux connected sockets or forwarding setups will break */
        if (sk && !inet_sk(sk)->inet_daddr)
                return;

--
Shawn

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