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

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

On Tue, Nov 30, 2021 at 10:53 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> 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.

Oh, and to be clear, I'm not proposing to bypass underlying perf
infra. Rather use it directly as an internal API, not through
perf_event_open syscall.

>
> 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(-)
> >
>
> [...]