Re: [PATCH 1/8] perf/kprobe: Add support to create multiple probes

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Nov 24, 2021 at 12:41 AM Jiri Olsa <jolsa@redhat.com> wrote:
>
> Adding support to create multiple probes within single perf event.
> This way we can associate single bpf program with multiple kprobes,
> because bpf program gets associated with the perf event.
>
> The perf_event_attr is not extended, current fields for kprobe
> attachment are used for multi attachment.

I'm a bit concerned with complicating perf_event_attr further to
support this multi-attach. For BPF, at least, we now have
bpf_perf_link and corresponding BPF_LINK_CREATE command in bpf()
syscall which allows much simpler and cleaner API to do this. Libbpf
will actually pick bpf_link-based attachment if kernel supports it. I
think we should better do bpf_link-based approach from the get go.

Another thing I'd like you to keep in mind and think about is BPF
cookie. Currently kprobe/uprobe/tracepoint allow to associate
arbitrary user-provided u64 value which will be accessible from BPF
program with bpf_get_attach_cookie(). With multi-attach kprobes this
because extremely crucial feature to support, otherwise it's both
expensive, inconvenient and complicated to be able to distinguish
between different instances of the same multi-attach kprobe
invocation. So with that, what would be the interface to specify these
BPF cookies for this multi-attach kprobe, if we are going through
perf_event_attr. Probably picking yet another unused field and
union-izing it with a pointer. It will work, but makes the interface
even more overloaded. While for LINK_CREATE we can just add another
pointer to a u64[] with the same size as number of kfunc names and
offsets.

But other than that, I'm super happy that you are working on these
complicated multi-attach capabilities! It would be great to benchmark
one-by-one attachment vs multi-attach to the same set of kprobes once
you arrive at the final implementation.

>
> For current kprobe atachment we use either:
>
>    kprobe_func (in config1) + probe_offset (in config2)
>
> to define kprobe by function name with offset, or:
>
>    kprobe_addr (in config2)
>
> to define kprobe with direct address value.
>
> For multi probe attach the same fields point to array of values
> with the same semantic. Each probe is defined as set of values
> with the same array index (idx) as:
>
>    kprobe_func[idx]  + probe_offset[idx]
>
> to define kprobe by function name with offset, or:
>
>    kprobe_addr[idx]
>
> to define kprobe with direct address value.
>
> The number of probes is passed in probe_cnt value, which shares
> the union with wakeup_events/wakeup_watermark values which are
> not used for kprobes.
>
> Since [1] it's possible to stack multiple probes events under
> one head event. Using the same code to allow that for probes
> defined under perf kprobe interface.
>
> [1] https://lore.kernel.org/lkml/156095682948.28024.14190188071338900568.stgit@devnote2/
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  include/uapi/linux/perf_event.h |   1 +
>  kernel/trace/trace_event_perf.c | 106 ++++++++++++++++++++++++++++----
>  kernel/trace/trace_kprobe.c     |  47 ++++++++++++--
>  kernel/trace/trace_probe.c      |   2 +-
>  kernel/trace/trace_probe.h      |   3 +-
>  5 files changed, 138 insertions(+), 21 deletions(-)
>

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