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

[Kuniyuki Iwashima <kuniyu@google.com>]

On Tue, 20 Oct 2026 09:50:32 -0400, Willem de Bruijn wrote:
> 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.

Agreed, FDO is critical especially for UDP, e.g. udp_rr
w/ 20000 flows will gain high performance boost 14Mpps -> 21Mpps.