Re: [PATCH bpf-next 4/7] libbpf: wire up spec management and other arch-independent USDT logic

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Mar 31, 2022 at 7:50 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>
> On Fri, 25 Mar 2022, Andrii Nakryiko wrote:
>
> > Last part of architecture-agnostic user-space USDT handling logic is to
> > set up BPF spec and, optionally, IP-to-ID maps from user-space.
> > usdt_manager performs a compact spec ID allocation to utilize
> > fixed-sized BPF maps as efficiently as possible. We also use hashmap to
> > deduplicate USDT arg spec strings and map identical strings to single
> > USDT spec, minimizing the necessary BPF map size. usdt_manager supports
> > arbitrary sequences of attachment and detachment, both of the same USDT
> > and multiple different USDTs and internally maintains a free list of
> > unused spec IDs. bpf_link_usdt's logic is extended with proper setup and
> > teardown of this spec ID free list and supporting BPF maps.
> >
>
> It might be good to describe the relationship between a USDT specification
> (spec) and the site specific targets that can be associated with it.  So
> the spec is the description of the provider + name + args, and the the
> target represents the potentially multiple sites associated with that
> spec.
>
> Specs are stored in the spec array map, indexed by spec_id; targets are
> stored in the ip_map, and these reference a spec id.  So from the BPF side
> we can use the bpf_cookie to look up the spec directly, or if cookies are
> not supported on the BPF side, we can look up ip -> spec_id mapping in
> ip_map, and from there can look up the spec_id -> spec in the spec map.
>

Correct, I'll incorporate that into comments I'm going to add in v2, thanks.

> Dumb question here: the spec id recycling is a lot of work;
> instead of maintaining this for the array map, couldn't we use a hashmap
> for spec ids with a monotonically-increasing next_spec_id value or
> something similar?

We could, but hashmap lookup is significantly slower than ARRAY
lookup, so I chose performance in this case. Maintaining the list of
IDs isn't that big of a deal.

>
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
>
> one suggestion below, but
>
> Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
>
> > ---
> >  tools/lib/bpf/usdt.c | 167 ++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 166 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
> > index 86d5d8390eb1..22f5f56992f8 100644
> > --- a/tools/lib/bpf/usdt.c
> > +++ b/tools/lib/bpf/usdt.c
>
> <snip>
>
> >               opts.ref_ctr_offset = target->sema_off;
> > +             opts.bpf_cookie = man->has_bpf_cookie ? spec_id : 0;
> >               uprobe_link = bpf_program__attach_uprobe_opts(prog, pid, path,
> >                                                             target->rel_ip, &opts);
> >               err = libbpf_get_error(link);
>
> should be uprobe_link I think.
>


Nice catch, will fix.