On Tue, Jun 8, 2021 at 4:07 PM Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > On Tue, Jun 8, 2021 at 2:07 PM Jiri Olsa <jolsa@xxxxxxxxxx> wrote: > > > > 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: > > > > 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@xxxxxxxxxx> 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. > > > > 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 > > yep. For every IP==btf_id there will be only two possible trampolines. > Should be easy enough to track and transition between them. > The standard fentry/fexit will only get negligible slowdown from > going through multi. > multi+fexit and fmod_ret needs to be thought through as well. > That's why I thought that 'ip' at the end should simplify things. Putting ip at the end has downsides. We might support >6 arguments eventually, at which point it will be super weird to have 6 args, ip, then the rest of arguments?.. Would it be too bad to put IP at -8 offset relative to ctx? That will also work for normal fentry/fexit, for which it's useful to have ip passed in as well, IMO. So no special casing for multi/non-multi, and it's backwards compatible. Ideally, I'd love it to be actually retrievable through a new BPF helper, something like bpf_caller_ip(ctx), but I'm not sure if we can implement this sanely, so I don't hold high hopes. > Only multi will have access to it. > But we can store it first too. fentry/fexit will see ctx=r1 with +8 offset > and will have normal args in ctx. Like ip isn't even there. > While multi trampoline is always doing ip, arg1,arg2, .., arg6 > and passes ctx = &ip into multi prog and ctx = &arg1 into fentry/fexit. > 'ret' for fexit is problematic though. hmm. > Maybe such clone multi trampoline for specific ip with 2 args will do: > ip, arg1, arg2, ret, 0, 0, 0, ret. > Then multi will have 6 args, though 3rd is actually ret. > Then fexit will have ret in the right place and multi prog will have > it as 7th arg.
