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

[Jason Xing]

On Tue, Jan 21, 2025 at 9:15 AM Jason Xing <kerneljasonxing@xxxxxxxxx> wrote:
>
> On Mon, Jan 20, 2025 at 9:20 PM Breno Leitao <leitao@xxxxxxxxxx> wrote:
> >
> >
> > On Mon, Jan 20, 2025 at 09:06:43PM +0800, Jason Xing wrote:
> > > On Mon, Jan 20, 2025 at 9:02 PM Breno Leitao <leitao@xxxxxxxxxx> wrote:
> > > > On Mon, Jan 20, 2025 at 08:08:52PM +0800, Jason Xing wrote:
> > > > > On Mon, Jan 20, 2025 at 8:03 PM Breno Leitao <leitao@xxxxxxxxxx> wrote:
> > > > > > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > > > > > index 4811727b8a02258ec6fa1fd129beecf7cbb0f90e..fc88c511e81bc12ec57e8dc3e9185a920d1bd079 100644
> > > > > > --- a/net/ipv4/tcp_input.c
> > > > > > +++ b/net/ipv4/tcp_input.c
> > > > > > @@ -2710,6 +2710,8 @@ 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(sk, newly_acked_sacked, newly_lost, flag);
> > > > > > +
> > > > >
> > > > > Are there any other reasons why introducing a new tracepoint here?
> > > > > AFAIK, it can be easily replaced by a bpf related program or script to
> > > > > monitor in the above position.
> > > >
> > > > In which position exactly?
> > >
> > > I meant, in the position where you insert a one-line tracepoint, which
> > > should be easily replaced with a bpf program (kprobe
> > > tcp_cwnd_reduction with two checks like in the earlier if-statement).
> > > It doesn't mean that I object to this new tracepoint, just curious if
> > > you have other motivations.
> >
> > This is exactly the current implementation we have at Meta, as it relies on
> > hooking into this specific function. This approach is unstable, as
> > compiler optimizations like inlining can break the functionality.
> >
> > This patch enhances the API's stability by introducing a guaranteed hook
> > point, allowing the compiler to make changes without disrupting the
> > BPF program's functionality.
>
> Surely it does :) The reason why I asked is that perhaps one year ago
> I'm trying to add many tracepoints so that the user space monitor can
> take advantage of these various stable hook points. I believe that
> there are many other places which might be inlined because of gcc,
> receiving a few similar reports in recent months, but there is no
> guarantee to not touch some functions which obviously break some
> monitor applications.

Before BPF gets widely used, changing function names will not hurt
applications. Things are a little bit different now, changing names or
inlined function will lead the monitor application adjusting codes
through versions, because they are not kabi functions.

Thanks,
Jason