linux-kernel - Re: [RFC] ftrace: Add support to keep some functions out of ftrace

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Date:   Tue, 16 Aug 2022 08:56:33 +0200
From:   Jiri Olsa <olsajiri@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Jiri Olsa <olsajiri@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [RFC] ftrace: Add support to keep some functions out of ftrace

On Mon, Aug 15, 2022 at 05:48:44PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 15, 2022 at 08:35:53AM -0700, Alexei Starovoitov wrote:
> > On Mon, Aug 15, 2022 at 8:28 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > On Mon, Aug 15, 2022 at 08:17:42AM -0700, Alexei Starovoitov wrote:
> > > > It's hiding a fake function from ftrace, since it's not a function
> > > > and ftrace infra shouldn't show it tracing logs.
> > > > In other words it's a _notrace_ function with nop5.
> > >
> > > Then make it a notrace function with a nop5 in it. That isn't hard.
> > 
> > That's exactly what we're trying to do.
> 
> All the while claiming ftrace is broken while it is not.
> 
> > Jiri's patch is one way to achieve that.
> 
> Fairly horrible way.
> 
> > What is your suggestion?
> 
> Mailed it already.
> 
> > Move it from C to asm ?
> 
> Would be much better than proposed IMO.

nice, that would be independent of the compiler atributes
and config checking..  will check on this one ;-)

thanks,
jirka

> 
> > Make it naked function with explicit inline asm?
> 
> Can be made to work but is iffy because the compiler can do horrible
> things with placing the asm().