On high speed links, the MPTCP-level receive buffer auto-tuning
happens with a frequency well above the TCP-level's one. That in
turn can cause excessive/unneeded receive buffer increase.

On such links, the initial rtt_us value is considerably higher
than the actual delay, but the current mptcp_rcv_space_adjust() logic
prevents msk->rcvq_space.rtt_us from decreasing.

Address the issue with a more accurate RTT estimation strategy: the
MPTCP-level RTT is set to the minimum of all the subflows, in a rcv-win
based interval, feeding data into the MPTCP-receive buffer.

Use some care to avoid updating msk and ssk level fields too often.

Fixes: a6b118febbab ("mptcp: add receive buffer auto-tuning")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
v1 -> v2:
  - do not use explicit reset flags - do rcv win based decision instead
  - discard 0 rtt_us samples from subflows
  - discard samples on non empty rx queue
  - discard "too high" samples, see the code comments WRT the whys
---
 include/trace/events/mptcp.h |  2 +-
 net/mptcp/protocol.c         | 74 ++++++++++++++++++++++--------------
 net/mptcp/protocol.h         |  7 +++-
 3 files changed, 53 insertions(+), 30 deletions(-)

diff --git a/include/trace/events/mptcp.h b/include/trace/events/mptcp.h
index 71fd6d33f48b..999133000cb8 100644
--- a/include/trace/events/mptcp.h
+++ b/include/trace/events/mptcp.h
@@ -212,7 +212,7 @@ TRACE_EVENT(mptcp_rcvbuf_grow,
 		__be32 *p32;
 
 		__entry->time = time;
-		__entry->rtt_us = msk->rcvq_space.rtt_us >> 3;
+		__entry->rtt_us = msk->rcv_rtt_est.rtt_us >> 3;
 		__entry->copied = msk->rcvq_space.copied;
 		__entry->inq = mptcp_inq_hint(sk);
 		__entry->space = msk->rcvq_space.space;
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index c85ad7ef29b0..414cca078541 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -868,6 +868,42 @@ static bool move_skbs_to_msk(struct mptcp_sock *msk, struct sock *ssk)
 	return moved;
 }
 
+static void mptcp_rcv_rtt_update(struct mptcp_sock *msk,
+				 struct mptcp_subflow_context *subflow)
+{
+	const struct tcp_sock *tp = tcp_sk(subflow->tcp_sock);
+	u32 rtt_us = tp->rcv_rtt_est.rtt_us;
+	u8 sr = tp->scaling_ratio;
+
+	/* MPTCP can react to incoming acks pushing data on different subflows,
+	 * causing apparent high RTT: ignore large samples; also do the update
+	 * only on RTT changes
+	 */
+	if (tp->rcv_rtt_est.seq == subflow->prev_rtt_seq ||
+	    (subflow->prev_rtt_us && (rtt_us >> 1) > subflow->prev_rtt_us))
+		return;
+
+	/* Similar to plain TCP, only consider samples with empty RX queue. */
+	if (!rtt_us || mptcp_data_avail(msk))
+		return;
+
+	/* Refresh the RTT after a full win per subflow */
+	subflow->prev_rtt_us = rtt_us;
+	subflow->prev_rtt_seq = tp->rcv_rtt_est.seq;
+	if (after(subflow->map_seq, msk->rcv_rtt_est.seq)) {
+		msk->rcv_rtt_est.seq = subflow->map_seq +
+				       tp->rcv_wnd * msk->pm.extra_subflows;
+		msk->rcv_rtt_est.rtt_us = rtt_us;
+		msk->scaling_ratio = sr;
+		return;
+	}
+
+	if (rtt_us < msk->rcv_rtt_est.rtt_us)
+		msk->rcv_rtt_est.rtt_us = rtt_us;
+	if (sr < msk->scaling_ratio)
+		msk->scaling_ratio = sr;
+}
+
 void mptcp_data_ready(struct sock *sk, struct sock *ssk)
 {
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
@@ -881,6 +917,7 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk)
 		return;
 
 	mptcp_data_lock(sk);
+	mptcp_rcv_rtt_update(msk, subflow);
 	if (!sock_owned_by_user(sk)) {
 		/* Wake-up the reader only for in-sequence data */
 		if (move_skbs_to_msk(msk, ssk) && mptcp_epollin_ready(sk))
@@ -2058,6 +2095,7 @@ static void mptcp_rcv_space_init(struct mptcp_sock *msk)
 	msk->rcvspace_init = 1;
 
 	mptcp_data_lock(sk);
+	msk->rcv_rtt_est.seq = atomic64_read(&msk->rcv_wnd_sent);
 	__mptcp_sync_rcvspace(sk);
 
 	/* Paranoid check: at least one subflow pushed data to the msk. */
@@ -2076,9 +2114,8 @@ static void mptcp_rcv_space_adjust(struct mptcp_sock *msk, int copied)
 {
 	struct mptcp_subflow_context *subflow;
 	struct sock *sk = (struct sock *)msk;
-	u8 scaling_ratio = U8_MAX;
-	u32 time, advmss = 1;
-	u64 rtt_us, mstamp;
+	u32 rtt_us, time;
+	u64 mstamp;
 
 	msk_owned_by_me(msk);
 
@@ -2093,29 +2130,8 @@ static void mptcp_rcv_space_adjust(struct mptcp_sock *msk, int copied)
 	mstamp = mptcp_stamp();
 	time = tcp_stamp_us_delta(mstamp, READ_ONCE(msk->rcvq_space.time));
 
-	rtt_us = msk->rcvq_space.rtt_us;
-	if (rtt_us && time < (rtt_us >> 3))
-		return;
-
-	rtt_us = 0;
-	mptcp_for_each_subflow(msk, subflow) {
-		const struct tcp_sock *tp;
-		u64 sf_rtt_us;
-		u32 sf_advmss;
-
-		tp = tcp_sk(mptcp_subflow_tcp_sock(subflow));
-
-		sf_rtt_us = READ_ONCE(tp->rcv_rtt_est.rtt_us);
-		sf_advmss = READ_ONCE(tp->advmss);
-
-		rtt_us = max(sf_rtt_us, rtt_us);
-		advmss = max(sf_advmss, advmss);
-		scaling_ratio = min(tp->scaling_ratio, scaling_ratio);
-	}
-
-	msk->rcvq_space.rtt_us = rtt_us;
-	msk->scaling_ratio = scaling_ratio;
-	if (time < (rtt_us >> 3) || rtt_us == 0)
+	rtt_us = READ_ONCE(msk->rcv_rtt_est.rtt_us);
+	if (rtt_us == U32_MAX || time < (rtt_us >> 3))
 		return;
 
 	if (msk->rcvq_space.copied <= msk->rcvq_space.space)
@@ -2941,7 +2957,8 @@ static void __mptcp_init_sock(struct sock *sk)
 	msk->scaling_ratio = TCP_DEFAULT_SCALING_RATIO;
 	msk->backlog_len = 0;
 	msk->rcvq_space.copied = 0;
-	msk->rcvq_space.rtt_us = 0;
+	msk->rcv_rtt_est.rtt_us = U32_MAX;
+	msk->scaling_ratio = U8_MAX;
 
 	WRITE_ONCE(msk->first, NULL);
 	inet_csk(sk)->icsk_sync_mss = mptcp_sync_mss;
@@ -3385,7 +3402,8 @@ static int mptcp_disconnect(struct sock *sk, int flags)
 	msk->bytes_retrans = 0;
 	msk->rcvspace_init = 0;
 	msk->rcvq_space.copied = 0;
-	msk->rcvq_space.rtt_us = 0;
+	msk->scaling_ratio = U8_MAX;
+	msk->rcv_rtt_est.rtt_us = U32_MAX;
 
 	/* for fallback's sake */
 	WRITE_ONCE(msk->ack_seq, 0);
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index adc0851bad69..051f21b06d33 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -340,11 +340,14 @@ struct mptcp_sock {
 				 */
 	struct mptcp_pm_data	pm;
 	struct mptcp_sched_ops	*sched;
+	struct {
+		u32	rtt_us;		/* Minimum RTT of subflows */
+		u64	seq;		/* Refresh RTT after this seq */
+	} rcv_rtt_est;
 	struct {
 		int	space;	/* bytes copied in last measurement window */
 		int	copied; /* bytes copied in this measurement window */
 		u64	time;	/* start time of measurement window */
-		u64	rtt_us; /* last maximum rtt of subflows */
 	} rcvq_space;
 	u8		scaling_ratio;
 	bool		allow_subflows;
@@ -523,6 +526,8 @@ struct mptcp_subflow_context {
 	u32	map_data_len;
 	__wsum	map_data_csum;
 	u32	map_csum_len;
+	u32	prev_rtt_us;
+	u32	prev_rtt_seq;
 	u32	request_mptcp : 1,  /* send MP_CAPABLE */
 		request_join : 1,   /* send MP_JOIN */
 		request_bkup : 1,
-- 
2.51.0