netdev - Re: [PATCH] net: Add tcp_drop

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Message-ID: <CANn89iKvr44ipuRYFaPTpzwz=B_+pgA94jsggQ946mjwreV6Aw@mail.gmail.com>
Date: Wed, 23 Oct 2024 17:43:46 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Yafang Shao <laoar.shao@...il.com>
Cc: davem@...emloft.net, dsahern@...nel.org, kuba@...nel.org, 
	pabeni@...hat.com, rostedt@...dmis.org, mhiramat@...nel.org, 
	mathieu.desnoyers@...icios.com, netdev@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, Menglong Dong <menglong8.dong@...il.com>
Subject: Re: [PATCH] net: Add tcp_drop_reason tracepoint

On Wed, Oct 23, 2024 at 4:35 PM Yafang Shao <laoar.shao@...il.com> wrote:
>
> On Wed, Oct 23, 2024 at 9:01 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Wed, Oct 23, 2024 at 2:33 PM Yafang Shao <laoar.shao@...il.com> wrote:
> > >
> > > We previously hooked the tcp_drop_reason() function using BPF to monitor
> > > TCP drop reasons. However, after upgrading our compiler from GCC 9 to GCC
> > > 11, tcp_drop_reason() is now inlined, preventing us from hooking into it.
> > > To address this, it would be beneficial to introduce a dedicated tracepoint
> > > for monitoring.
> >
> > This patch would require changes in user space tracers.
> > I am surprised no one came up with a noinline variant.
> >
> > __bpf_kfunc is using
> >
> > #define __bpf_kfunc __used __retain noinline
> >
> > I would rather not have include/trace/events/tcp.h becoming the
> > biggest file in TCP stack...
>
> I’d prefer not to introduce a new tracepoint if we can easily hook it
> with BPF. Does the following change look good to you?
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 092456b8f8af..ebea844cc974 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -4720,7 +4720,7 @@ static bool tcp_ooo_try_coalesce(struct sock *sk,
>         return res;
>  }
>
> -static void tcp_drop_reason(struct sock *sk, struct sk_buff *skb,
> +noinline static void tcp_drop_reason(struct sock *sk, struct sk_buff *skb,
>                             enum skb_drop_reason reason)
>  {
>         sk_drops_add(sk, skb);
>

I would suggest adding an explicit keyword, like the one we have for
noinline_for_stack, for documentation purposes.

noinline_for_tracing perhaps ?

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 1a957ea2f4fe78ed12d7f6a65e5759d07cea4449..9a687ca4bb4392583d150349ee11015bcb82ec74
100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -265,6 +265,12 @@ struct ftrace_likely_data {
  */
 #define noinline_for_stack noinline

+/*
+ * Use noinline_for_tracing for functions that should not be inlined,
+ * for tracing reasons.
+ */
+#define noinline_for_tracing noinline
+
 /*
  * Sanitizer helper attributes: Because using __always_inline and
  * __no_sanitize_* conflict, provide helper attributes that will either expand