[PATCH mptcp-next 3/3] DO-NOT-MERGE: mptcp: sched: penalise counters

Instrumentation for validating the two preceding patches; not for merge.

Adds two MPTcpExt SNMP counters:
- CwndPenalized: times a subflow cwnd was actually halved;
- PenalCandidate: times the rate trigger picked a slow subflow.

Together they separate "the guard held the penalty back" (PenalCandidate
high, CwndPenalized ~0) from "the trigger never fired" (both ~0), which is
what the receive-window-limited case needs to be read correctly.

Co-developed-by: Matthieu Baerts (NGI0) <[email protected]>
Signed-off-by: Shardul Bankar <[email protected]>
---
 net/mptcp/mib.c      | 2 ++
 net/mptcp/mib.h      | 2 ++
 net/mptcp/protocol.c | 8 ++++++++
 3 files changed, 12 insertions(+)

diff --git a/net/mptcp/mib.c b/net/mptcp/mib.c
index d9bd4f4afcc0..1299613e183b 100644
--- a/net/mptcp/mib.c
+++ b/net/mptcp/mib.c
@@ -88,6 +88,8 @@ static const struct snmp_mib mptcp_snmp_list[] = {
 	SNMP_MIB_ITEM("BacklogDrop", MPTCP_MIB_BACKLOGDROP),
 	SNMP_MIB_ITEM("RcvPruned", MPTCP_MIB_RCVPRUNED),
 	SNMP_MIB_ITEM("OfoPruned", MPTCP_MIB_OFO_PRUNED),
+	SNMP_MIB_ITEM("CwndPenalized", MPTCP_MIB_CWNDPENALIZED),
+	SNMP_MIB_ITEM("PenalCandidate", MPTCP_MIB_PENALCAND),
 };
 
 /* mptcp_mib_alloc - allocate percpu mib counters
diff --git a/net/mptcp/mib.h b/net/mptcp/mib.h
index 18f35f7e0a2d..93b3a7e9584f 100644
--- a/net/mptcp/mib.h
+++ b/net/mptcp/mib.h
@@ -91,6 +91,8 @@ enum linux_mptcp_mib_field {
 	MPTCP_MIB_BACKLOGDROP,		/* Backlog over memory limit */
 	MPTCP_MIB_RCVPRUNED,		/* Dropped due to memory constrains */
 	MPTCP_MIB_OFO_PRUNED,		/* MPTCP-level OoO queue pruned */
+	MPTCP_MIB_CWNDPENALIZED,	/* DEBUG: subflow cwnd halved by the scheduler (#345) */
+	MPTCP_MIB_PENALCAND,		/* DEBUG: picker chose a slow (low-rate) subflow */
 	__MPTCP_MIB_MAX
 };
 
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 7cbc5aa17e22..80866f09831a 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1607,6 +1607,7 @@ static void mptcp_penalise_cwnd(struct sock *ssk)
 	subflow->penalise = false;
 	subflow->last_penalise = tcp_jiffies32;
 	tcp_snd_cwnd_set(tp, max_t(u32, cwnd >> 1, 2));
+	MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_CWNDPENALIZED);
 	if (cwnd >= tp->snd_ssthresh)
 		tp->snd_ssthresh = max_t(u32, tp->snd_ssthresh >> 1, 2);
 }
@@ -1694,6 +1695,13 @@ struct sock *mptcp_subflow_get_send(struct mptcp_sock *msk)
 	 * once per RTT.
 	 */
 	subflow = mptcp_subflow_ctx(ssk);
+	/* DEBUG: count how often the trigger picks a slow path, so a gated-off
+	 * run (PenalCandidate high, CwndPenalized 0) is distinguishable from one
+	 * where the trigger never fired.
+	 */
+	if (fastest && ssk != fastest &&
+	    (u64)subflow->avg_pacing_rate * MPTCP_PENALISE_RATE_RATIO < max_pace)
+		MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_PENALCAND);
 	subflow->penalise = fastest && ssk != fastest &&
 			    (u64)subflow->avg_pacing_rate * MPTCP_PENALISE_RATE_RATIO < max_pace &&
 			    inet_csk(ssk)->icsk_ca_state == TCP_CA_Open &&

-- 
2.34.1