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

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Message-ID: <YvoVgMzMuQbAEayk@krava>
Date:   Mon, 15 Aug 2022 11:44:32 +0200
From:   Jiri Olsa <olsajiri@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Jiri Olsa <olsajiri@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        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 10:03:17AM +0200, Peter Zijlstra wrote:
> On Sat, Aug 13, 2022 at 03:02:52PM -0400, Steven Rostedt wrote:
> > On Fri, 12 Aug 2022 23:18:15 +0200
> > Jiri Olsa <olsajiri@...il.com> wrote:
> > 
> > > the patch below moves the bpf function into sepatate object and switches
> > > off the -mrecord-mcount for it.. so the function gets profile call
> > > generated but it's not visible to ftrace
> 
> Why ?!?

there's bpf dispatcher code that updates bpf_dispatcher_xdp_func
function with bpf_arch_text_poke and that can race with ftrace update
if the function is traced

the idea to solve it is to 'mark' the function independent of ftrace,
and add a way to make the function invissible to ftrace but with the
profile code fentry call generated

jirka