rxrpc: Fix congestion control algorithm

Make the following fixes to the congestion control algorithm:

 (1) Don't vary the cwnd starting value by the size of RXRPC_TX_SMSS since
     that's currently held constant - set to the size of a jumbo subpacket
     payload so that we can create jumbo packets on the fly.  The current
     code invariably picks 3 as the starting value.

     Further, the starting cwnd needs to be an even number because we ack
     every other packet, so set it to 4.

 (2) Don't cut ssthresh when we see an ACK come from the peer with a
     receive window (rwind) less than ssthresh.  ssthresh keeps track of
     characteristics of the connection whereas rwind may be reduced by the
     peer for any reason - and may be reduced to 0.

Fixes: 1fc4fa2a ("rxrpc: Fix congestion management")
Fixes: 08511150 ("rxrpc: Reduce ssthresh to peer's receive window")
Signed-off-by: David Howells <dhowells@redhat.com>
Suggested-by: Simon Wilkinson <sxw@auristor.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Reviewed-by: Jeffrey Altman &lt;jaltman@auristor.com <mailto:jaltman@auristor.com&gt;>
Link: https://lore.kernel.org/r/20240503150749.1001323-2-dhowells@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/rxrpc/ar-internal.h

@@ -697,7 +697,7 @@ struct rxrpc_call {
 	 * packets) rather than bytes.
 	 */
 #define RXRPC_TX_SMSS		RXRPC_JUMBO_DATALEN
-#define RXRPC_MIN_CWND		(RXRPC_TX_SMSS > 2190 ? 2 : RXRPC_TX_SMSS > 1095 ? 3 : 4)
+#define RXRPC_MIN_CWND		4
 	u8			cong_cwnd;	/* Congestion window size */
 	u8			cong_extra;	/* Extra to send for congestion management */
 	u8			cong_ssthresh;	/* Slow-start threshold */

net/rxrpc/call_object.c

@@ -174,12 +174,7 @@ struct rxrpc_call *rxrpc_alloc_call(struct rxrpc_sock *rx, gfp_t gfp,
 	call->rx_winsize = rxrpc_rx_window_size;
 	call->tx_winsize = 16;
 
-	if (RXRPC_TX_SMSS > 2190)
-		call->cong_cwnd = 2;
-	else if (RXRPC_TX_SMSS > 1095)
-		call->cong_cwnd = 3;
-	else
-		call->cong_cwnd = 4;
+	call->cong_cwnd = RXRPC_MIN_CWND;
 	call->cong_ssthresh = RXRPC_TX_MAX_WINDOW;
 
 	call->rxnet = rxnet;

net/rxrpc/input.c

@@ -685,9 +685,6 @@ static void rxrpc_input_ack_trailer(struct rxrpc_call *call, struct sk_buff *skb
 		call->tx_winsize = rwind;
 	}
 
-	if (call->cong_ssthresh > rwind)
-		call->cong_ssthresh = rwind;
-
 	mtu = min(ntohl(trailer->maxMTU), ntohl(trailer->ifMTU));
 
 	peer = call->peer;