On Thu, Mar 31, 2022 at 7:50 AM Alan Maguire <alan.maguire@xxxxxxxxxx> 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@xxxxxxxxxx> > > one suggestion below, but > > Reviewed-by: Alan Maguire <alan.maguire@xxxxxxxxxx> > > > --- > > 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.
