linux-kernel - Re: [PATCH net-next] udp: Add tracepoint for udp

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Message-ID: <680122de92908_166f4f2942a@willemb.c.googlers.com.notmuch>
Date: Thu, 17 Apr 2025 11:48:46 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Song Liu <songliubraving@...a.com>, 
 Paolo Abeni <pabeni@...hat.com>
Cc: Breno Leitao <leitao@...ian.org>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Steven Rostedt <rostedt@...dmis.org>, 
 Masami Hiramatsu <mhiramat@...nel.org>, 
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
 "David S. Miller" <davem@...emloft.net>, 
 David Ahern <dsahern@...nel.org>, 
 Eric Dumazet <edumazet@...gle.com>, 
 "kuba@...nel.org" <kuba@...nel.org>, 
 Simon Horman <horms@...nel.org>, 
 "kuniyu@...zon.com" <kuniyu@...zon.com>, 
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>, 
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
 "linux-trace-kernel@...r.kernel.org" <linux-trace-kernel@...r.kernel.org>, 
 "yonghong.song@...ux.dev" <yonghong.song@...ux.dev>, 
 "song@...nel.org" <song@...nel.org>, 
 Kernel Team <kernel-team@...a.com>
Subject: Re: [PATCH net-next] udp: Add tracepoint for udp_sendmsg()

Song Liu wrote:
> Hi Paolo, 
> 
> > On Apr 17, 2025, at 6:17 AM, Paolo Abeni <pabeni@...hat.com> wrote:
> > 
> > On 4/17/25 1:34 PM, Breno Leitao wrote:
> >> On Thu, Apr 17, 2025 at 08:57:24AM +0200, Paolo Abeni wrote:
> >>> On 4/16/25 9:23 PM, Breno Leitao wrote:
> >>>> Add a lightweight tracepoint to monitor UDP send message operations,
> >>>> similar to the recently introduced tcp_sendmsg_locked() trace event in
> >>>> commit 0f08335ade712 ("trace: tcp: Add tracepoint for
> >>>> tcp_sendmsg_locked()")
> >>> 
> >>> Why is it needed? what would add on top of a plain perf probe, which
> >>> will be always available for such function with such argument, as the
> >>> function can't be inlined?
> >> 
> >> Why this function can't be inlined?
> > 
> > Because the kernel need to be able find a pointer to it:
> > 
> > .sendmsg = udp_sendmsg,
> > 
> > I'll be really curious to learn how the compiler could inline that.
> 
> It is true that functions that are only used via function pointers
> will not be inlined by compilers (at least for those we have tested).
> For this reason, we do not worry about functions in various
> tcp_congestion_ops. However, udp_sendmsg is also called directly
> by udpv6_sendmsg, so it can still get inlined by LTO. 
> 
> Thanks,
> Song
> 

I would think that hitting this tracepoint for ipv6_addr_v4mapped
addresses is unintentional and surprising, as those would already
hit udpv6_sendmsg.

On which note, any IPv4 change to UDP needs an equivalent IPv6 one.