Merge branch 'rxrpc-implement-jumbo-data-transmission-and-rack-tlp'

David Howells says:

====================
rxrpc: Implement jumbo DATA transmission and RACK-TLP

Here's a series of patches to implement two main features:

 (1) The transmission of jumbo data packets whereby several DATA packets of
     a particular size can be glued together into a single UDP packet,
     allowing us to make use of larger MTU sizes.  The basic jumbo
     subpacket capacity is 1412 bytes (RXRPC_JUMBO_DATALEN) and, say, an
     MTU of 8192 allows five of them to be transmitted as one.

     An alternative (and possibly more efficient way) would be to
     expand/shrink the capacity of each DATA packet to match the MTU and
     thus save on header and tail-gap overhead, but the Rx protocol does
     not provide a mechanism for splitting the data - especially as the
     transported data is encrypted per-packet - and so UDP fragmentation
     would be the only way to handle this.

     In fact, in the future, AF_RXRPC also needs to look at shrinking the
     packet size where the MTU is smaller - for instance in the case of
     being carried by IPv6 over wifi where there isn't capacity for a 1412
     byte capacity.

 (2) RACK-TLP to manage packet loss and retransmission in conjunction with
     the congestion control algorithm.

These allow for better data throughput and work towards being able to have
larger transmission windows.

To this end, the following changes are also made:

 (1) Use a single large array of kvec structs for the I/O thread rather
     than having one per transmission buffer.  We need a much bigger
     collection of kvecs for ping padding

 (2) Implement path-MTU probing by sending padded PING ACK packets and
     monitoring for PING RESPONSE ACKs.  The pmtud value determined is used
     to configure the construction of jumbo DATA packets.

 (3) The transmission queue is changed from a linked list of transmission
     buffer structs to a linked list of transmission-queue structs, each of
     which points to either 32 or 64 transmission buffers (depending on cpu
     word size) and various bits of metadata are concentrated in the queue
     structs rather than the buffers to make better use of the cpu cache.

 (4) SACK data is stored in the transmission-queue structures in batches of
     32 or 64 making it faster to process rather than being spread amongst
     all the individual packet buffers.

 (5) Don't change the DF flag on the UDP socket unless we need to - and
     basically only enable it for path-MTU probing.

There are also some additional bits:

 (1) Fix the handling of connection aborts to poke the aborted connections.

 (2) Don't set the MORE-PACKETS Rx header flag on the wire.  No one
     actually checks it and it is, in any case, generated inconsistently
     between implementations.

 (3) Request an ACK when, during call transmission, there's a stall in the
     app generating the data to be transmitted.

 (4) Fix attention starvation in the I/O thread by making sure we go
     through all outstanding events rather than returning to the beginning
     of the check cycle after any time we process an event.

 (5) Don't use the skbuff timestamp in the calculation of timeouts and RTT
     as we really should include local processing time in that too.
     Further, getting receive skbuff timestamps may be expensive.

 (6) Make RTT tracking per call with the saving of the value between calls,
     even within the same connection channel.  The initial call timeout
     starts off large to allow the server time to set up its state before
     the initial reply.

 (7) Don't allocate txbuf structs for ACK packets, but rather use page
     frags and MSG_SPLICE_PAGES.

 (8) Use irq-disabling locks for interactions between app threads and I/O
     threads so that the I/O thread doesn't get help up.

 (9) Make rxrpc set the REQUEST-ACK flag on an outgoing packet when cwnd is
     at RXRPC_MIN_CWND (currently 4), not at 2 which it can never reach.

(10) Add some tracing bits and pieces (including displaying the userStatus
     field in an ACK header) and some more stats counters (including
     different sizes of jumbo packets sent/received).

Link: https://lore.kernel.org/r/20240306000655.1100294-1-dhowells@redhat.com/
====================

Link: https://patch.msgid.link/20241204074710.990092-1-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>