Re: [PATCH] ipv4: Add sysctl knob to control early socket demux

From: Alexander Duyck <redacted>
Date: Thu, 21 Jun 2012 16:58:31 -0700

This change is meant to add a control for disabling early socket demux.
The main motivation behind this patch is to provide an option to disable
the feature as it adds an additional cost to routing that reduces overall
throughput by up to 5%.  For example one of my systems went from 12.1Mpps
to 11.6 after the early socket demux was added.  It looks like the reason
for the regression is that we are now having to perform two lookups, first
the one for an established socket, and then the one for the routing table.

By adding this patch and toggling the value for ip_early_demux to 0 I am
able to get back to the 12.1Mpps I was previously seeing.

Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Alexander Duyck <redacted>

I applied this for now, making a minor change to move the local
variables down into the new basic block you created.

There has got to be a way to make this really cheap.  At the very
least we can have the GRO code store away the ports and therefore
allow us to just do a direct call to try and demux the socket.  Thus,
we'd avoid all of pskb_may_pull() et al. packet validations, and
packet header pointer calculations.

Furthermore, we can reduce to overhead by making a special inet
established hash demux that doesn't check for time-wait sockets,
reducing the number of probes to 1 from 2.