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.
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
net/mptcp/sockopt.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index f44b364b0055..453d6c78c25c 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -1450,6 +1450,12 @@ void mptcp_sockopt_sync_locked(struct mptcp_sock *msk, struct sock *ssk)
msk_owned_by_me(msk);
+ /* 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);
--
2.41.0