[PATCH net-next] trace: tcp: Add tracepoint for tcp_cwnd_reduction()

Add a lightweight tracepoint to monitor TCP congestion window
adjustments via tcp_cwnd_reduction(). This tracepoint enables tracking
of:
- TCP window size fluctuations
- Active socket behavior
- Congestion window reduction events

Meta has been using BPF programs to monitor this function for years.
Adding a proper tracepoint provides a stable API for all users who need
to monitor TCP congestion window behavior.

Use DECLARE_TRACE instead of TRACE_EVENT to avoid creating trace event
infrastructure and exporting to tracefs, keeping the implementation
minimal. (Thanks Steven Rostedt)

Signed-off-by: Breno Leitao <leitao@debian.org>
---
Changes since RFC:
- Change from a full tracepoint to DECLARE_TRACE() as suggested by
  Steven
- Link to RFC: https://lore.kernel.org/r/20250120-cwnd_tracepoint-v1-1-36b0e0d643fa@debian.org (local)
---
 include/trace/events/tcp.h | 5 +++++
 net/ipv4/tcp_input.c       | 2 ++
 2 files changed, 7 insertions(+)

diff --git a/include/trace/events/tcp.h b/include/trace/events/tcp.h
index a27c4b619dffd7dcc72fffa71bf0fd5e34fe6681..d574e6151dc4f7430206f9ccefe0bf0d463aaa52 100644
--- a/include/trace/events/tcp.h
+++ b/include/trace/events/tcp.h

@@ -259,6 +259,11 @@ TRACE_EVENT(tcp_retransmit_synack,
 		  __entry->saddr_v6, __entry->daddr_v6)
 );
 
+DECLARE_TRACE(tcp_cwnd_reduction_tp,
+	TP_PROTO(const struct sock *sk, int newly_acked_sacked,
+		 int newly_lost, int flag),
+	TP_ARGS(sk, newly_acked_sacked, newly_lost, flag));
+
 #include <trace/events/net_probe_common.h>
 
 TRACE_EVENT(tcp_probe,

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index eb82e01da911048b41ca380f913ef55566be79a7..1a667e67df6beacde9871a50d44e180c2943ded0 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c

@@ -2710,6 +2710,8 @@ void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked, int newly_lost,
 	if (newly_acked_sacked <= 0 || WARN_ON_ONCE(!tp->prior_cwnd))
 		return;
 
+	trace_tcp_cwnd_reduction_tp(sk, newly_acked_sacked, newly_lost, flag);
+
 	tp->prr_delivered += newly_acked_sacked;
 	if (delta < 0) {
 		u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered +

---
base-commit: 09717c28b76c30b1dc8c261c855ffb2406abab2e
change-id: 20250120-cwnd_tracepoint-2e11c996a9cb

Best regards,
-- 
Breno Leitao [off-list ref]