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

On Tue, Jun 08, 2021 at 11:49:03AM -0700, Alexei Starovoitov wrote:

On Tue, Jun 08, 2021 at 08:17:00PM +0200, Jiri Olsa wrote:

quoted

On Tue, Jun 08, 2021 at 08:42:32AM -0700, Alexei Starovoitov wrote:

quoted

On Sat, Jun 5, 2021 at 4:11 AM Jiri Olsa [off-list ref] wrote:

quoted

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.

ok, so we'd clone multi trampoline if there's request to attach
standard trampoline to some IP from multi trampoline

.. and transform currently attached standard trampoline for IP
into clone of multi trampoline, if there's request to create
multi trampoline that covers that IP

jirka