Merge branch 'mptcp-improve-mptcp-level-window-tracking'

Mat Martineau says:

====================
mptcp: Improve MPTCP-level window tracking

This series improves MPTCP receive window compliance with RFC 8684 and
helps increase throughput on high-speed links. Note that patch 3 makes a
change in tcp_output.c

For the details, Paolo says:

I've been chasing bad/unstable performance with multiple subflows
on very high speed links.

It looks like the root cause is due to the current mptcp-level
congestion window handling. There are apparently a few different
sub-issues:

- the rcv_wnd is not effectively shared on the tx side, as each
  subflow takes in account only the value received by the underlaying
  TCP connection. This is addressed in patch 1/5

- The mptcp-level offered wnd right edge is currently allowed to shrink.
  Reading section 3.3.4.:

"""
   The receive window is relative to the DATA_ACK.  As in TCP, a
   receiver MUST NOT shrink the right edge of the receive window (i.e.,
   DATA_ACK + receive window).  The receiver will use the data sequence
   number to tell if a packet should be accepted at the connection
   level.
"""

I read the above as we need to reflect window right-edge tracking
on the wire, see patch 4/5.

- The offered window right edge tracking can happen concurrently on
  multiple subflows, but there is no mutex protection. We need an
  additional atomic operation - still patch 4/5

This series additionally bumps a few new MIBs to track all the above
(ensure/observe that the suspected races actually take place).

I could not access again the host where the issue was so
noticeable, still in the current setup the tput changes from
[6-18] Gbps to 19Gbps very stable.
====================

Link: https://lore.kernel.org/r/20220504215408.349318-1-mathew.j.martineau@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>