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

From: Jiri Olsa <olsajiri@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Jiri Olsa <olsajiri@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, bpf <bpf@vger.kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [RFC] ftrace: Add support to keep some functions out of ftrace
Date: Thu, 18 Aug 2022 23:14:32 +0200	[thread overview]
Message-ID: <Yv6ruI/UFqztRaD2@krava> (raw)
In-Reply-To: <20220818165024.433f56fd@gandalf.local.home>

On Thu, Aug 18, 2022 at 04:50:24PM -0400, Steven Rostedt wrote:
> On Thu, 18 Aug 2022 22:27:07 +0200
> Jiri Olsa <olsajiri@gmail.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		    \
>    ...
> 

is there a reason to keep NON_MCOUNT_PATCHABLE section for x86?  otherwise LGTM

jirka