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From:   Jiri Olsa <olsajiri@gmail.com>
Date:   Mon, 12 Jun 2023 22:25:52 +0200
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Yonghong Song <yhs@meta.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Andrii Nakryiko <andrii@kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        linux-trace-kernel@vger.kernel.org, bpf@vger.kernel.org,
        Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        Jackie Liu <liu.yun@linux.dev>
Subject: Re: [PATCHv2] ftrace: Show all functions with addresses in
 available_filter_functions_addrs
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On Mon, Jun 12, 2023 at 11:02:22AM -0400, Steven Rostedt wrote:
> On Mon, 12 Jun 2023 07:49:53 -0700
> Yonghong Song <yhs@meta.com> wrote:
> 
> > I am actually interested in how available_filter_functions_addrs
> > will be used. For example, bpf_program__attach_kprobe_multi_opts()
> > can already take addresses from kallsyms. How to use
> > available_filter_functions_addrs to facilitate kprobe_multi?

the problem is that we need to do 2 passes:

 - through available_filter_functions and find out if the function is traceable
 - through /proc/kallsyms to get the address for traceable function

having available_filter_functions symbols together with addresses allow
us to skip the kallsyms step

and we are ok with the address in available_filter_functions_addr not being the
function entry, because kprobe_multi uses fprobe and that handles both entry and
patch-site address properly

> > Do we need to change kernel APIs? It would be great at least we
> > got a RFC patch to answer these questions.
> 
> I agree, having that information would also be useful to me.
> 
> Jiri? Andrii?

so we have 2 interfaces how to create kprobe_multi link:

  a) passing symbols to kernel

     1) user gathers symbols and need to ensure that they are
        trace-able -> pass through available_filter_functions file

     2) kernel takes those symbols and translates them to addresses
        through kallsyms api

     3) addresses are passed to fprobe/ftrace through:

         register_fprobe_ips
         -> ftrace_set_filter_ips

  b) passing addresses to kernel

     1) user gathers symbols and needs to ensure that they are
        trace-able -> pass through available_filter_functions file

     2) user takes those symbols and translates them to addresses
       through /proc/kallsyms

     3) addresses are passed to the kernel and kernel calls:

         register_fprobe_ips
         -> ftrace_set_filter_ips


The new available_filter_functions_addrs file helps us with option b),
because we can make 'b 1' and 'b 2' in one step - while filtering traceable
functions, we get the address directly.

I tested the new available_filter_functions_addrs changes with some hacked
selftest changes, you can check it in here [1].

I assume Jackie Liu will send new version of her patchset [2] based on this
new available_filter_functions_addrs file.

I think we should have these changes coming together and add some perf
measurements from before and after to make the benefit apparent.

jirka


[1] https://git.kernel.org/pub/scm/linux/kernel/git/jolsa/perf.git/commit/?h=bpf/avail_addrs&id=fecaeeaf40bae034715ab2e9a46ca1dc16371e8e
[2] https://lore.kernel.org/bpf/20230526155026.1419390-1-liu.yun@linux.dev/#r