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

On Fri, Nov 14, 2014 at 2:16 PM, Eric Dumazet [off-list ref] wrote:

On Fri, 2014-11-14 at 12:16 -0800, Andy Lutomirski wrote:

quoted

Sure, but how do I know that it'll be the same CPU next time?

Because the NIC always use same RX queue for a given flow.

So if you setup your IRQ affinities properly, the same CPU will drain
packets from this RX queue. And since RFS is off, you have the guarantee
the same CPU will be used to process packets in TCP stack.

Right.  My concern is that if RFS is off, everything works fine, but
if RFS is on (and the app has no good way to know), then silly results
will happen.  I think I'd rather have the getsockopt fail if RFS is on
or at least give some indication so that the app can react
accordingly.

--Andy

This SO_INCOMING_CPU info is a hint, there is no guarantee eg if you use
bonding and some load balancer or switch decides to send packets on
different links.

Most NIC use Toeplitz hash, so given the 4-tuple, and rss key (40
bytes), you can actually compute the hash in software and know on which
RX queue traffic should land.

-- 
Andy Lutomirski
AMA Capital Management, LLC