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

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Date:   Tue, 8 Jun 2021 16:05:29 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>, Daniel Xu <dxu@...uu.xyz>,
        Viktor Malik <vmalik@...hat.com>
Subject: Re: [PATCH 13/19] bpf: Add support to link multi func tracing program

On Tue, Jun 8, 2021 at 2:07 PM Jiri Olsa <jolsa@...hat.com> 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@...nel.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.
>
> 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.
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.