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

[Eric Dumazet <edumazet@google.com>]

On Mon, Feb 24, 2025 at 8:13 PM Yonghong Song <yhs@meta.com> wrote:
>
> > ________________________________________
> >
> > On Mon, Feb 24, 2025 at 7:24 PM Breno Leitao <leitao@debian.org> wrote:
> >>
> >> 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 {
> > }
>
> In LTO mode, tcp_sendmsg could be inlined cross files. For example,
>
>   net/ipv4/tcp.c:
>        int tcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
>   net/ipv4/tcp_bpf.c:
>        ...
>       return tcp_sendmsg(sk, msg, size);
>   net/ipv6/af_inet6.c:
>        ...
>        return INDIRECT_CALL_2(prot->sendmsg, tcp_sendmsg, udpv6_sendmsg, ...)
>
> And this does happen in our production environment.

And we do not have a way to make the kprobe work even if LTO decided
to inline a function ?

This seems like a tracing or LTO issue, this could be addressed there
in a generic way
and avoid many other patches to work around this.