netdev - Re: [PATCH] net: Add tcp_drop

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Message-ID: <CALOAHbDiev7-mosZL+1D8N4vr4pkJTPdQAdR4+GkOYS6BygSPQ@mail.gmail.com>
Date: Thu, 24 Oct 2024 10:21:55 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Eric Dumazet <edumazet@...gle.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 11:43 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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 ?

Good suggestion! This approach eliminates the need to add comments for
each noinline.

>
> 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



--
Regards
Yafang