[PATCH 6.8 217/336] rxrpc: Fix congestion control algorithm

Greg Kroah-Hartman
2024-05-14T10:47:18Z

From: David Howells <dhowells@redhat.com>

[ Upstream commit ba4e103848d3a2a28a0445e39f4a9564187efe54 ]

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@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 <jaltman@auristor.com <mailto:jaltman@auristor.com>>
Link: https://lore.kernel.org/r/20240503150749.1001323-2-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@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 7818aae1be8e0..4301dd20b4eaa 100644
--- a/net/rxrpc/ar-internal.h
+++ b/net/rxrpc/ar-internal.h
@@ -692,7 +692,7 @@ 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 9fc9a6c3f6858..3847b14af7f3c 100644
--- a/net/rxrpc/call_object.c
+++ b/net/rxrpc/call_object.c
@@ -175,12 +175,7 @@ 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 718ffd184ddb6..f7304e06aadca 100644
--- a/net/rxrpc/input.c
+++ b/net/rxrpc/input.c
@@ -688,9 +688,6 @@ 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;
--
2.43.0