tipc: add smart nagle feature

tipc: add smart nagle feature

We introduce a feature that works like a combination of TCP_NAGLE and
TCP_CORK, but without some of the weaknesses of those. In particular,
we will not observe long delivery delays because of delayed acks, since
the algorithm itself decides if and when acks are to be sent from the
receiving peer.

- The nagle property as such is determined by manipulating a new
  'maxnagle' field in struct tipc_sock. If certain conditions are met,
  'maxnagle' will define max size of the messages which can be bundled.
  If it is set to zero no messages are ever bundled, implying that the
  nagle property is disabled.
- A socket with the nagle property enabled enters nagle mode when more
  than 4 messages have been sent out without receiving any data message
  from the peer.
- A socket leaves nagle mode whenever it receives a data message from
  the peer.

In nagle mode, messages smaller than 'maxnagle' are accumulated in the
socket write queue. The last buffer in the queue is marked with a new
'ack_required' bit, which forces the receiving peer to send a CONN_ACK
message back to the sender upon reception.

The accumulated contents of the write queue is transmitted when one of
the following events or conditions occur.

- A CONN_ACK message is received from the peer.
- A data message is received from the peer.
- A SOCK_WAKEUP pseudo message is received from the link level.
- The write queue contains more than 64 1k blocks of data.
- The connection is being shut down.
- There is no CONN_ACK message to expect. I.e., there is currently
  no outstanding message where the 'ack_required' bit was set. As a
  consequence, the first message added after we enter nagle mode
  is always sent directly with this bit set.

This new feature gives a 50-100% improvement of throughput for small
(i.e., less than MTU size) messages, while it might add up to one RTT
to latency time when the socket is in nagle mode.

Acked-by: Ying Xue <ying.xue@windreiver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>