Re: [PATCH] udp: Add noinline_for_tracing annotation for udp{,_v6}_send_skb()

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Jason Xing wrote:
> On Tue, Oct 20, 2026 at 7:08 AM Josh Don <joshdon@google.com> wrote:
> >
> > We hook udp_send_skb() and udp_v6_send_skb() using BPF fentry programs
> > to monitor UDP transmits. Both functions are static and have two
> > callers: udp{,v6}_sendmsg() and udp{,_v6}_push_pending_frames().
> >
> > After an FDO profile update, udp_v6_send_skb() is now inlined into
> > udpv6_sendmsg(), while an out-of-line copy remains for
> > udp_v6_push_pending_frames(). The fentry program still attaches
> > successfully, but it is silently bypassed for all uncorked sends.
> >
> > To address this, make both functions noinline explicitly for tracing,
> > as was done for tcp_drop_reason() in commit dbd5e2e79ed8 ("net: tcp:
> > Add noinline_for_tracing annotation for tcp_drop_reason()").
> >
> > Assisted-by: LLM
> > Signed-off-by: Josh Don <joshdon@google.com>
> 
> I actually mentioned this thing a few times before. The original
> thread was from Breno who tried to apply a similar change for tcp
> sendmsg. The fact is until now we don't guarantee those functions
> should be kept noinline like how uAPI works just for BPF prog. What I
> suggested was to select a series of functions in one Doc so that we
> could tell people those functions should be noinline or whatever else
> just for tracing in a soft way?
> 
> Otherwise people would argue there are more functions that should be
> done in the same way because they internally trace it.
> 
> Willem, Kuniyuki, any thoughts?

We'll have to review hooks on a case-by-case basis for performance
impact.

If possible, it's preferable to select hooks that cannot be inlined.
Over limiting what the compiler is allowed to do.