[net-next,8/9] mptcp: ignore notsent_lowat setting at the subflow level

Commit Message

Mat Martineau Oct. 23, 2023, 8:44 p.m. UTC

From: Paolo Abeni <[e-mail]>

Any latency related tuning taking action at the subflow level does
not really affect the user-space, as only the main MPTCP socket is
relevant.

Anyway any limiting setting may foul the MPTCP scheduler, not being
able to fully use the subflow-level cwin, leading to very poor b/w
usage.

Enforce notsent_lowat to be a no-op on every subflow.

Note that TCP_NOTSENT_LOWAT is currently not supported, and properly
dealing with that will require more invasive changes.

Reviewed-by: Mat Martineau <[e-mail]>
Signed-off-by: Paolo Abeni <[e-mail]>
Signed-off-by: Mat Martineau <[e-mail]>
---
 net/mptcp/sockopt.c | 6 ++++++
 1 file changed, 6 insertions(+)

Patch

diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index abf0645cb65d..72858d7d8974 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -1452,6 +1452,12 @@  void mptcp_sockopt_sync_locked(struct mptcp_sock *msk, struct sock *ssk)
 
 	ssk->sk_rcvlowat = 0;
 
+	/* subflows must ignore any latency-related settings: will not affect
+	 * the user-space - only the msk is relevant - but will foul the
+	 * mptcp scheduler
+	 */
+	tcp_sk(ssk)->notsent_lowat = UINT_MAX;
+
 	if (READ_ONCE(subflow->setsockopt_seq) != msk->setsockopt_seq) {
 		sync_socket_options(msk, ssk);