On Tue, Nov 30, 2021 at 10:53 PM Andrii Nakryiko
[off-list ref] wrote:

On Wed, Nov 24, 2021 at 12:41 AM Jiri Olsa [off-list ref] wrote:

quoted

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.

quoted

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/ (local)
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(-)

[...]