Re: [PATCH 13/19] bpf: Add support to link multi func tracing program

Alexei Starovoitov
2021-06-08T18:54:27Z

On Tue, Jun 08, 2021 at 08:17:00PM +0200, Jiri Olsa wrote:
> On Tue, Jun 08, 2021 at 08:42:32AM -0700, Alexei Starovoitov wrote:
> > On Sat, Jun 5, 2021 at 4:11 AM Jiri Olsa <jolsa@kernel.org> wrote:
> > >
> > > Adding support to attach multiple functions to tracing program
> > > by using the link_create/link_update interface.
> > >
> > > Adding multi_btf_ids/multi_btf_ids_cnt pair to link_create struct
> > > API, that define array of functions btf ids that will be attached
> > > to prog_fd.
> > >
> > > The prog_fd needs to be multi prog tracing program (BPF_F_MULTI_FUNC).
> > >
> > > The new link_create interface creates new BPF_LINK_TYPE_TRACING_MULTI
> > > link type, which creates separate bpf_trampoline and registers it
> > > as direct function for all specified btf ids.
> > >
> > > The new bpf_trampoline is out of scope (bpf_trampoline_lookup) of
> > > standard trampolines, so all registered functions need to be free
> > > of direct functions, otherwise the link fails.
> >
> > Overall the api makes sense to me.
> > The restriction of multi vs non-multi is too severe though.
> > The multi trampoline can serve normal fentry/fexit too.
>
> so multi trampoline gets called from all the registered functions,
> so there would need to be filter for specific ip before calling the
> standard program.. single cmp/jnz might not be that bad, I'll check

You mean reusing the same multi trampoline for all IPs and regenerating
it with a bunch of cmp/jnz checks? There should be a better way to scale.
Maybe clone multi trampoline instead?
IPs[1-10] will point to multi.
IP[11] will point to a clone of multi that serves multi prog and
fentry/fexit progs specific for that IP.