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>

2 files changed, 8 insertions, 0 deletions

diff --git a/include/net/sctp/stream_sched.h b/include/net/sctp/stream_sched.h
index fa00dc20a0d7..913170710adb 100644
--- a/include/net/sctp/stream_sched.h
+++ b/include/net/sctp/stream_sched.h
@@ -58,5 +58,6 @@ void sctp_sched_ops_register(enum sctp_sched_type sched,
 			     struct sctp_sched_ops *sched_ops);
 void sctp_sched_ops_prio_init(void);
 void sctp_sched_ops_rr_init(void);
+void sctp_sched_ops_fc_init(void);
 
 #endif /* __sctp_stream_sched_h__ */
diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
index e1f6e7fc2b11..2f1c9f50b352 100644
--- a/include/net/sctp/structs.h
+++ b/include/net/sctp/structs.h
@@ -1429,6 +1429,10 @@ struct sctp_stream_out_ext {
 		struct {
 			struct list_head rr_list;
 		};
+		struct {
+			struct list_head fc_list;
+			__u32 fc_length;
+		};
 	};
 };
 
@@ -1475,6 +1479,9 @@ struct sctp_stream {
 			/* The next stream in line */
 			struct sctp_stream_out_ext *rr_next;
 		};
+		struct {
+			struct list_head fc_list;
+		};
 	};
 	struct sctp_stream_interleave *si;
 };