tipc: reduce risk of user starvation during link congestion

The socket code currently handles link congestion by either blocking
and trying to send again when the congestion has abated, or just
returning to the user with -EAGAIN and let him re-try later.

This mechanism is prone to starvation, because the wakeup algorithm is
non-atomic. During the time the link issues a wakeup signal, until the
socket wakes up and re-attempts sending, other senders may have come
in between and occupied the free buffer space in the link. This in turn
may lead to a socket having to make many send attempts before it is
successful. In extremely loaded systems we have observed latency times
of several seconds before a low-priority socket is able to send out a
message.

In this commit, we simplify this mechanism and reduce the risk of the
described scenario happening. When a message is attempted sent via a
congested link, we now let it be added to the link's backlog queue
anyway, thus permitting an oversubscription of one message per source
socket. We still create a wakeup item and return an error code, hence
instructing the sender to block or stop sending. Only when enough space
has been freed up in the link's backlog queue do we issue a wakeup event
that allows the sender to continue with the next message, if any.

The fact that a socket now can consider a message sent even when the
link returns a congestion code means that the sending socket code can
be simplified. Also, since this is a good opportunity to get rid of the
obsolete 'mtu change' condition in the three socket send functions, we
now choose to refactor those functions completely.

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

1 files changed, 3 insertions, 3 deletions

diff --git a/net/tipc/bcast.c b/net/tipc/bcast.c
index aa1babbea385..c35fad3e08e8 100644
--- a/net/tipc/bcast.c
+++ b/net/tipc/bcast.c
@@ -174,7 +174,7 @@ static void tipc_bcbase_xmit(struct net *net, struct sk_buff_head *xmitq)
  *                    and to identified node local sockets
  * @net: the applicable net namespace
  * @list: chain of buffers containing message
- * Consumes the buffer chain, except when returning -ELINKCONG
+ * Consumes the buffer chain.
  * Returns 0 if success, otherwise errno: -ELINKCONG,-EHOSTUNREACH,-EMSGSIZE
  */
 int tipc_bcast_xmit(struct net *net, struct sk_buff_head *list)
@@ -197,7 +197,7 @@ int tipc_bcast_xmit(struct net *net, struct sk_buff_head *list)
 	tipc_bcast_unlock(net);
 
 	/* Don't send to local node if adding to link failed */
-	if (unlikely(rc)) {
+	if (unlikely(rc && (rc != -ELINKCONG))) {
 		__skb_queue_purge(&rcvq);
 		return rc;
 	}
@@ -206,7 +206,7 @@ int tipc_bcast_xmit(struct net *net, struct sk_buff_head *list)
 	tipc_bcbase_xmit(net, &xmitq);
 	tipc_sk_mcast_rcv(net, &rcvq, &inputq);
 	__skb_queue_purge(list);
-	return 0;
+	return rc;
 }
 
 /* tipc_bcast_rcv - receive a broadcast packet, and deliver to rcv link