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

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Message-ID: <4D934267-EE73-49DB-BEAF-7550945A38C9@fb.com>
Date: Thu, 17 Apr 2025 15:37:59 +0000
From: Song Liu <songliubraving@...a.com>
To: 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()

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