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

Breno Leitao
2025-04-02T12:57:34Z

On Wed, Feb 26, 2025 at 11:31:49AM -0700, David Ahern wrote:
> On 2/26/25 11:27 AM, Eric Dumazet wrote:
>
> ie., moving the tracepoint to tcp_sendmsg_locked should solve the inline
> problem. From there, the question is inside the loop or at entry to the
> function. Inside the loop has been very helpful for me.

I am happy to get it inside the loop. I am planning to send the
following patch when the MW opens. How does it sound?

trace: tcp: Add tracepoint for tcp_sendmsg_locked()

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

Meta has been using BPF programs to monitor tcp_sendmsg() 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.

David Ahern is using a similar functionality with a custom patch[1]. So,
this means we have more than a single use case for this request.

The implementation adopts David's approach[1] for greater flexibility
compared to the initial proposal.

Link: https://lore.kernel.org/all/70168c8f-bf52-4279-b4c4-be64527aa1ac@kernel.org/ [1]
Signed-off-by: Breno Leitao <leitao@debian.org>

diff --git a/include/trace/events/tcp.h b/include/trace/events/tcp.h
index 1a40c41ff8c30..cd90a8c66d683 100644
--- a/include/trace/events/tcp.h
+++ b/include/trace/events/tcp.h
@@ -259,6 +259,29 @@ TRACE_EVENT(tcp_retransmit_synack,
__entry->saddr_v6, __entry->daddr_v6)
);

+TRACE_EVENT(tcp_sendmsg_locked,
+ TP_PROTO(struct msghdr *msg, struct sk_buff *skb, int size_goal),
+
+ TP_ARGS(msg, skb, size_goal),
+
+ TP_STRUCT__entry(
+ __field(__u64, skb)
+ __field(int, skb_len)
+ __field(int, msg_left)
+ __field(int, size_goal)
+ ),
+
+ TP_fast_assign(
+ __entry->skb = (__u64)skb;
+ __entry->skb_len = skb ? skb->len : 0;
+ __entry->msg_left = msg_data_left(msg);
+ __entry->size_goal = size_goal;
+ ),
+
+ TP_printk("skb %llx skb_len %d msg_left %d size_goal %d", __entry->skb,
+ __entry->skb_len, __entry->msg_left, __entry->size_goal)
+);
+
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 ea8de00f669d0..822cd40ce2b7f 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1160,6 +1160,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
if (skb)
copy = size_goal - skb->len;

+ trace_tcp_sendmsg_locked(msg, skb, size_goal);
+
if (copy <= 0 || !tcp_skb_can_collapse_to(skb)) {
bool first_skb;