linux-kernel - [PATCH net 1/5] rxrpc: Fix congestion control algorithm

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Date: Fri,  3 May 2024 16:07:39 +0100
From: David Howells <dhowells@...hat.com>
To: netdev@...r.kernel.org
Cc: David Howells <dhowells@...hat.com>,
	Marc Dionne <marc.dionne@...istor.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	linux-afs@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Simon Wilkinson <sxw@...istor.com>
Subject: [PATCH net 1/5] 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: 1fc4fa2ac93d ("rxrpc: Fix congestion management")
Fixes: 0851115090a3 ("rxrpc: Reduce ssthresh to peer's receive window")
Signed-off-by: David Howells <dhowells@...hat.com>
Suggested-by: Simon Wilkinson <sxw@...istor.com>
cc: Marc Dionne <marc.dionne@...istor.com>
cc: "David S. Miller" <davem@...emloft.net>
cc: Eric Dumazet <edumazet@...gle.com>
cc: Jakub Kicinski <kuba@...nel.org>
cc: Paolo Abeni <pabeni@...hat.com>
cc: linux-afs@...ts.infradead.org
cc: netdev@...r.kernel.org
---
 net/rxrpc/ar-internal.h | 2 +-
 net/rxrpc/call_object.c | 7 +------
 net/rxrpc/input.c       | 3 ---
 3 files changed, 2 insertions(+), 10 deletions(-)

diff --git a/net/rxrpc/ar-internal.h b/net/rxrpc/ar-internal.h
index 08c0a32db8c7..08de24658f4f 100644
--- a/net/rxrpc/ar-internal.h
+++ b/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 */
diff --git a/net/rxrpc/call_object.c b/net/rxrpc/call_object.c
index 01fa71e8b1f7..f9e983a12c14 100644
--- a/net/rxrpc/call_object.c
+++ b/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;
diff --git a/net/rxrpc/input.c b/net/rxrpc/input.c
index 3dedb8c0618c..860075f1290b 100644
--- a/net/rxrpc/input.c
+++ b/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;