Re: [PATCH net-next] net: introduce SO_INCOMING_CPU

On Fri, 2014-11-14 at 14:10 -0800, Andy Lutomirski wrote:

I have a bunch of threads that are pinned to various CPUs or groups of
CPUs.  Each thread is responsible for a fixed set of flows.  I'd like
those flows to go to those CPUs.

RFS will eventually do it, but it would be nice if I could
deterministically ask for a flow to be routed to the right CPU.  Also,
if my thread bounces temporarily to another CPU, I don't really need
the flow to follow it -- I'd like it to stay put.

This has a significant benefit over using automatic steering: with
automatic steering, I have to make all of the hash tables have a size
around the square of the total number of the flows in order to make it
reliable.

Something like SO_STEER_TO_THIS_CPU would be fine, as long as it
reported whether it worked (for my diagnostics).

This requires some kind of hardware support, and unfortunately this is
not generic.

With SO_INCOMING_CPU, you simply can pass fd of sockets around threads,
so that a dumb RSS multiqueue NIC is OK (assuming you are not using some
encapsulation that NIC is not able to parse to find L4 information)

Steering is a dream, I really think its easier to build flows so that
their RX queue matches your requirements.

We usually can pick at least one element of the 4-tuple, so its actually
possible to get this before connect().


Two cases :

1) Passive connections.

   After accept(), get SO_INCOMING_CPU, then pass the fd to appropriate
thread of your pool.

2) Active connections .
   find a proper 4-tuple, bind() then connect(). Eventually check
SO_INCOMING_CPU to verify your expectations.