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

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Nov 30, 2021 at 10:53:58PM -0800, Andrii Nakryiko 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.

I'm not sure we could bypass perf event easily.. perhaps introduce
BPF_PROG_TYPE_RAW_KPROBE as we did for tracepoints or just new
type for multi kprobe attachment like BPF_PROG_TYPE_MULTI_KPROBE
that might be that way we'd have full control over the API

> 
> 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.

I have the change for bpftrace to use this and even though there's
some speed up, it's not as substantial as for trampolines

looks like we 'only' got rid of the multiple perf syscall overheads,
compared to rcu syncs timeouts like we eliminated for trampolines 

I'll make full benchmarks once we have some final solution

jirka