Re: [PATCH bpf-next 2/2] perf: stop using deprecated bpf__object_next() API

[Jiri Olsa <jolsa@redhat.com>]

On Thu, Jan 06, 2022 at 09:54:38AM -0800, Christy Lee wrote:
> Thank you so much, I was able to reproduce the original tests after applying
> the bug fix. I will submit a new patch set with the more detailed comments.
> 
> The only deprecated functions that need to be removed after this would be
> bpf_program__set_prep() (how perf sets the bpf prologue) and
> bpf_program__nth_fd() (how perf leverages multi instance bpf). They look a
> little more involved and I'm not sure how to approach those. Jiri, would you
> mind taking a look at those please?

hi,
I checked and here's the way perf uses this interface:

  - when bpf object/sources is specified on perf command line
    we use bpf_object__open to load it

  - user can define parameters in the section name for each bpf program
    like:

      SEC("lock_page=__lock_page page->flags")
      int lock_page(struct pt_regs *ctx, int err, unsigned long flags)
      {
             return 1;
      }

    which tells perf to 'prepare' some extra bpf code for the program,
    like to put value of 'page->flags' into 'flags' argument above

  - perf generates extra prologue code to retrieve this data and does
    that before the program is loaded by using bpf_program__set_prep
    callback

  - now the reason why we use bpf_program__set_prep for that, is because
    it allows to create multiple instances for one bpf program

  - we need multiple instances for single program, because probe can
    result in multiple attach addresses (like for inlined functions)
    with possible different ways of getting the arguments we need
    to load

I guess you want to get rid of that whole 'struct instances' related
stuff, is that right?

perf would need to load all the needed instances for program manually
and somehow bypass/workaround bpf_object__load.. is there a way to
manually add extra programs to bpf_object?

thoughts? ;-)

thanks,
jirka