Re: [PATCH net-next] udp: Add tracepoint for udp_sendmsg()

[Willem de Bruijn]

Song Liu wrote:
> 
> 
> > On Apr 17, 2025, at 8:48 AM, Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx> wrote:
> > 
> > Song Liu wrote:
> >> Hi Paolo, 
> >> 
> >>> On Apr 17, 2025, at 6:17 AM, Paolo Abeni <pabeni@xxxxxxxxxx> 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.
> 
> It is up to the user to decide how these tracepoints should be 
> used. For example, the user may only be interested in 
> udpv6_sendmsg => udp_sendmsg case. Without a tracepoint, the user
> has to understand whether the compiler inlined this function. 
> 
> > 
> > On which note, any IPv4 change to UDP needs an equivalent IPv6 one.
> 
> Do you mean we need to also add tracepoints for udpv6_sendmsg?

If there is consensus that a tracepoint at this point is valuable,
then it should be supported equally for IPv4 and IPv6.

That holds true for all such hooks. No IPv4 only.