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

Hello David,

On Mon, Feb 24, 2025 at 12:16:04PM -0700, David Ahern wrote:

On 2/24/25 12:03 PM, Eric Dumazet wrote:

quoted

On Mon, Feb 24, 2025 at 7:24 PM Breno Leitao [off-list ref] wrote:

quoted

Add a lightweight tracepoint to monitor TCP sendmsg operations, enabling
the tracing of TCP messages being sent.

Meta has been using BPF programs to monitor this function for years,
indicating significant interest in observing this important
functionality. Adding a proper tracepoint provides a stable API for all
users who need visibility into TCP message transmission.

The implementation uses DECLARE_TRACE instead of TRACE_EVENT to avoid
creating unnecessary trace event infrastructure and tracefs exports,
keeping the implementation minimal while stabilizing the API.

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_sendmsg_tp {
                ....
        }

I would expect tcp_sendmsg() being stable enough ?

kprobe:tcp_sendmsg {
}

Also, if a tracepoint is added, inside of tcp_sendmsg_locked would cover
more use cases (see kernel references to it).

Agree, this seems to provide more useful information

We have a patch for a couple years now with a tracepoint inside the

Sorry, where do you have this patch? is it downstream?

while (msg_data_left(msg)) {
}

loop which is more useful than just entry to sendmsg.

Do you mean something like the following?

diff --git a/include/trace/events/tcp.h b/include/trace/events/tcp.h
index 1a40c41ff8c30..23318e252d6b9 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_sendmsg_tp,
+	TP_PROTO(const struct sock *sk, const struct msghdr *msg, size_t size, ssize_t copied),
+	TP_ARGS(sk, msg, size, copied)
+);
+
 DECLARE_TRACE(tcp_cwnd_reduction_tp,
 	TP_PROTO(const struct sock *sk, int newly_acked_sacked,
 		 int newly_lost, int flag),

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 08d73f17e8162..5fcef82275d4a 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c

@@ -1290,6 +1290,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 			sk_mem_charge(sk, copy);
 		}
 
+		trace_tcp_sendmsg_tp(sk, msg, size, copy);
+
 		if (!copied)
 			TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_PSH;