sctp: add fair capacity stream scheduler

As it says in rfc8260#section-3.5 about the fair capacity scheduler:

   A fair capacity distribution between the streams is used.  This
   scheduler considers the lengths of the messages of each stream and
   schedules them in a specific way to maintain an equal capacity for
   all streams.  The details are implementation dependent.  interleaving
   user messages allows for a better realization of the fair capacity
   usage.

This patch adds Fair Capacity Scheduler based on the foundations added
by commit 5bbbbe32a431 ("sctp: introduce stream scheduler foundations"):

A fc_list and a fc_length are added into struct sctp_stream_out_ext and
a fc_list is added into struct sctp_stream. In .enqueue, when there are
chunks enqueued into a stream, this stream will be linked into stream->
fc_list by its fc_list ordered by its fc_length. In .dequeue, it always
picks up the 1st skb from stream->fc_list. In .dequeue_done, fc_length
is increased by chunk's len and update its location in stream->fc_list
according to the its new fc_length.

Note that when the new fc_length overflows in .dequeue_done, instead of
resetting all fc_lengths to 0, we only reduced them by U32_MAX / 4 to
avoid a moment of imbalance in the scheduling, as Marcelo suggested.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/net/sctp/stream_sched.c b/net/sctp/stream_sched.c
index 330067002deb..1ebd14ef8daa 100644
--- a/net/sctp/stream_sched.c
+++ b/net/sctp/stream_sched.c
@@ -124,6 +124,7 @@ void sctp_sched_ops_init(void)
 	sctp_sched_ops_fcfs_init();
 	sctp_sched_ops_prio_init();
 	sctp_sched_ops_rr_init();
+	sctp_sched_ops_fc_init();
 }
 
 static void sctp_sched_free_sched(struct sctp_stream *stream)