[PATCH net-next v2] trace: tcp: Add tracepoint for tcp_cwnd_reduction()

[Breno Leitao]

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)

Given that this patch creates a rawtracepoint, you could hook into it
using regular tooling, like bpftrace, using regular rawtracepoint
infrastructure, such as:

	rawtracepoint:tcp_cwnd_reduction_tp {
		....
	}

Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
---
Changes in v2:
- Close the parenthesis in a new line to honor the tcp.h format (Jakub).
- Add the bpftrace example in the commit message (Jakub)
- Link to v1: https://lore.kernel.org/r/20250207-cwnd_tracepoint-v1-1-13650f3ca96d@xxxxxxxxxx

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@xxxxxxxxxx
---
 include/trace/events/tcp.h | 6 ++++++
 net/ipv4/tcp_input.c       | 2 ++
 2 files changed, 8 insertions(+)

diff --git a/include/trace/events/tcp.h b/include/trace/events/tcp.h
index a27c4b619dffd7dcc72fffa71bf0fd5e34fe6681..1a40c41ff8c30a31b5c7002a4109de1cd8ef389e 100644
--- a/include/trace/events/tcp.h
+++ b/include/trace/events/tcp.h
@@ -259,6 +259,12 @@ 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 4686783b70defe0457571efd72d41ac88c528f7b..3f79718fdb1ec7a4352dc691147da527448c1f46 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2709,6 +2709,8 @@ 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: 7a7e0197133d18cfd9931e7d3a842d0f5730223f
change-id: 20250120-cwnd_tracepoint-2e11c996a9cb

Best regards,
-- 
Breno Leitao <leitao@xxxxxxxxxx>