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

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Message-ID: <CAADnVQ+n=x=CuBk23UNnD9CHVXjrXLUofbockh-SWaLwH3H9fw@mail.gmail.com>
Date:   Thu, 18 Aug 2022 14:00:21 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Jiri Olsa <olsajiri@...il.com>,
        Peter Zijlstra <peterz@...radead.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 Thu, Aug 18, 2022 at 1:50 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Thu, 18 Aug 2022 22:27:07 +0200
> Jiri Olsa <olsajiri@...il.com> wrote:
>
> > ok, so the problem with __attribute__((patchable_function_entry(5))) is that
> > it puts function address into __patchable_function_entries section, which is
> > one of ftrace locations source:
> >
> >   #define MCOUNT_REC()    . = ALIGN(8);     \
> >     __start_mcount_loc = .;                 \
> >     KEEP(*(__mcount_loc))                   \
> >     KEEP(*(__patchable_function_entries))   \
> >     __stop_mcount_loc = .;                  \
> >    ...
> >
> >
> > it looks like __patchable_function_entries is used for other than x86 archs,
> > so we perhaps we could have x86 specific MCOUNT_REC macro just with
> > __mcount_loc section?
>
> So something like this:
>
> #ifdef CONFIG_X86
> # define NON_MCOUNT_PATCHABLE KEEP(*(__patchable_function_entries))
> # define MCOUNT_PATCHABLE
> #else
> # define NON_MCOUNT_PATCHABLE
> # define MCOUNT_PATCHABLE  KEEP(*(__patchable_function_entries))
> #endif
>
>   #define MCOUNT_REC()    . = ALIGN(8);     \
>     __start_mcount_loc = .;                 \
>     KEEP(*(__mcount_loc))                   \
>     MCOUNT_PATCHABLE                        \
>     __stop_mcount_loc = .;                  \
>     NON_MCOUNT_PATCHABLE                    \
>    ...
>
> ??

That's what more or less Peter's patch is doing:
Here it is again for reference:
https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?id=8d075bdf11193f1d276bf19fa56b4b8dfe24df9e