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

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 22 Jul 2022 13:08:11 +0200
Jiri Olsa <jolsa@kernel.org> wrote:

> hi,
> we recently hit bug where ftrace update raced with bpf_dispatcher_update
> that calls directly bpf_arch_text_poke [1].
> 
> The bpf_dispatcher_update creates special trampoline and attaches it to
> designated bpf_dispatcher_xdp_func function, which is run for xdp bpf
> programs from several places.
> 
> After discussion with Alexei we'd rather keep this code update out of
> ftrace, because it's already slow and had troubles with CI because of
> that.
> 
> This patch is presenting the idea to allow some functions not to be
> managed by ftrace by marking them with NOFTRACE_SYMBOL macro and
> such symbols will not be added to ftrace_pages on the kernel start.

NACK on any generic way to hide mcount/fentry functions from ftrace.

There's a lot of infrastructure to see what functions are being
modified, as the user should know. (See tracefs/enabled_functions).

There's no need for a generic way to hide functions. Once that happens,
it will grow and then it will be more confusing to why some functions are
traced while others are not.

> 
> Please note it's RFC so I did not bother with some fast search for
> is_noftrace_function function.
> 
> Perhaps we could use existing NOKPROBE_SYMBOL for this? but I'm not
> sure you can (or want) to run function trace on such symbols.

I trace those functions all the time. Yes, I want to continue doing so.

-- Steve