[PATCH net-next] net/tcp: Add explicit tracepoint for tcp_syn_ack_timeout()

[PATCH net-next] net/tcp: Add explicit tracepoint for tcp_syn_ack_timeout()

[Emil Tsalapatis]

Clang can inline the tcp_syn_ack_timeout() function during compilation,
making it impossible to use kprobes for tracing without preventing
inlining. Add an explicit tracepoint to it instead.

Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
---
 include/trace/events/tcp.h | 72 ++++++++++++++++++++++++++++++++++++++
 net/ipv4/tcp_timer.c       |  3 ++
 2 files changed, 75 insertions(+)

diff --git a/include/trace/events/tcp.h b/include/trace/events/tcp.h
index f155f95cdb6e..37ffaa50faad 100644
--- a/include/trace/events/tcp.h
+++ b/include/trace/events/tcp.h
@@ -332,6 +332,78 @@ TRACE_EVENT(tcp_retransmit_synack,
 		  __entry->saddr_v6, __entry->daddr_v6)
 );
 
+TRACE_EVENT(tcp_syn_ack_timeout,
+
+	TP_PROTO(const struct request_sock *req),
+
+	TP_ARGS(req),
+
+	TP_STRUCT__entry(
+		__field(const void *, req)
+		__field(__u16, sport)
+		__field(__u16, dport)
+		__field(__u16, family)
+		__array(__u8, saddr, 4)
+		__array(__u8, daddr, 4)
+		__array(__u8, saddr_v6, 16)
+		__array(__u8, daddr_v6, 16)
+		__field(u8, state)
+		__field(u8, num_timeout)
+		__field(bool, acked)
+	),
+
+
+	TP_fast_assign(
+		const struct inet_request_sock *ireq = inet_rsk(req);
+		__be32 *p32;
+
+		__entry->req = req;
+
+		__entry->sport = ireq->ir_num;
+		__entry->dport = ntohs(ireq->ir_rmt_port);
+		__entry->family = req->__req_common.skc_family;
+
+		p32 = (__be32 *) __entry->saddr;
+		*p32 = ireq->ir_loc_addr;
+
+		p32 = (__be32 *) __entry->daddr;
+		*p32 = ireq->ir_rmt_addr;
+
+#if IS_ENABLED(CONFIG_IPV6)
+		/*
+		 * Cannot use TP_STORE_ADDRS directly because it assumes
+		 * there is an sk available.
+		 */
+		if (__entry->family == AF_INET6) {
+			struct in6_addr *pin6;
+
+			pin6 = (struct in6_addr *)__entry->saddr_v6;
+			*pin6 = ireq->ir_v6_loc_addr;
+			pin6 = (struct in6_addr *)__entry->daddr_v6;
+			*pin6 = ireq->ir_v6_rmt_addr;
+		} else {
+			TP_STORE_V4MAPPED(__entry, ireq->ir_loc_addr, ireq->ir_rmt_addr);
+		}
+#else
+		TP_STORE_V4MAPPED(__entry, ireq->ir_loc_addr, ireq->ir_rmt_addr);
+#endif
+
+		__entry->state		= ireq->ireq_state;
+		__entry->num_timeout	= req->num_timeout;
+		__entry->acked		= ireq->acked;
+	),
+
+	TP_printk("family=%s sport=%hu dport=%hu saddr=%pI4 "
+		"daddr=%pI4 saddrv6=%pI6c daddrv6=%pI6c "
+		"ireq_state=%s num_timeout=%u acked=%d",
+		  show_family_name(__entry->family),
+		  __entry->sport, __entry->dport,
+		  __entry->saddr, __entry->daddr,
+		  __entry->saddr_v6, __entry->daddr_v6,
+		  show_tcp_state_name(__entry->state),
+		  __entry->num_timeout, __entry->acked)
+);
+
 TRACE_EVENT(tcp_sendmsg_locked,
 	TP_PROTO(const struct sock *sk, const struct msghdr *msg,
 		 const struct sk_buff *skb, int size_goal),
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index bf171b5e1eb3..8f482a6b43e3 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -24,6 +24,7 @@
 #include <net/tcp.h>
 #include <net/tcp_ecn.h>
 #include <net/rstreason.h>
+#include <trace/events/tcp.h>
 
 static u32 tcp_clamp_rto_to_user_timeout(const struct sock *sk)
 {
@@ -753,6 +754,8 @@ void tcp_syn_ack_timeout(const struct request_sock *req)
 	struct net *net = read_pnet(&inet_rsk(req)->ireq_net);
 
 	__NET_INC_STATS(net, LINUX_MIB_TCPTIMEOUTS);
+
+	trace_tcp_syn_ack_timeout(req);
 }
 
 void tcp_reset_keepalive_timer(struct sock *sk, unsigned long len)
-- 
2.54.0

Re: [PATCH net-next] net/tcp: Add explicit tracepoint for tcp_syn_ack_timeout()

[Eric Dumazet]

On Mon, Jul 6, 2026 at 6:01 PM Emil Tsalapatis <emil@etsalapatis.com> wrote:
>
> Clang can inline the tcp_syn_ack_timeout() function during compilation,
> making it impossible to use kprobes for tracing without preventing
> inlining. Add an explicit tracepoint to it instead.

So much copy/pasting for a very small issue :/

>
> Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
> ---
>  include/trace/events/tcp.h | 72 ++++++++++++++++++++++++++++++++++++++
>  net/ipv4/tcp_timer.c       |  3 ++
>  2 files changed, 75 insertions(+)
>

tcp_syn_ack_timeout() is hardly a fast path, so you can instead:

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 322db13333c7..ab2c3de19e46 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -748,7 +748,7 @@ static void tcp_write_timer(struct timer_list *t)
        sock_put(sk);
 }

-void tcp_syn_ack_timeout(const struct request_sock *req)
+noinline_for_tracing void tcp_syn_ack_timeout(const struct request_sock *req)
 {
        struct net *net = read_pnet(&inet_rsk(req)->ireq_net);