Re: [PATCH net-next] net/tcp: Prevent inlining tcp_syn_ack_timeout()

[Jakub Kicinski <kuba@kernel.org>]

On Tue, 21 Jul 2026 15:24:52 +0200 Paolo Abeni wrote:
> On 7/8/26 8:08 PM, Emil Tsalapatis wrote:
> > The tcp_syn_ack_timeout() function gets inlined by Clang,
> > preventing tracing. Since the call is not in the fast
> > path, prevent it from being inlined.
> > 
> > Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
> > ---
> >  net/ipv4/tcp_timer.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> > index bf171b5e1eb3..f7215d53bbda 100644
> > --- a/net/ipv4/tcp_timer.c
> > +++ b/net/ipv4/tcp_timer.c
> > @@ -748,7 +748,7 @@ static void tcp_write_timer(struct timer_list *t)
> >  	sock_put(sk);
> >  }
> >  
> > -void tcp_syn_ack_timeout(const struct request_sock *req)
> > +noinline_for_tracing void tcp_syn_ack_timeout(const struct request_sock *req)
> >  {
> >  	struct net *net = read_pnet(&inet_rsk(req)->ireq_net);
> >    
> 
> What's the point of tracing such function? It just increment a mib. If
> you want to discriminate between TFO and non TFO syn ack timeout,
> possibly adding another MIB counter and incrementing it in
> tcp_fastopen_synack_timer() would be better???

FWIW we seem to capture the 5 tuple when it happens, not just mib
counter. Internal commit (form 2019) just says we want to know when 
syn-ack is blackholed, unclear under what conditions this happens.