Re: [PATCHv3 bpf-next 09/13] libbpf: Add bpf_program__attach_kprobe_multi_opts function

Jiri Olsa
2022-03-18T09:08:33Z

On Thu, Mar 17, 2022 at 08:53:15PM -0700, Alexei Starovoitov wrote:
> On Wed, Mar 16, 2022 at 5:26 AM Jiri Olsa <jolsa@kernel.org> wrote:
> > +
> > +struct bpf_link *
> > +bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
> > + const char *pattern,
> > + const struct bpf_kprobe_multi_opts *opts)
> > +{
> > + LIBBPF_OPTS(bpf_link_create_opts, lopts);
> > + struct kprobe_multi_resolve res = {
> > + .pattern = pattern,
> > + };
> > + struct bpf_link *link = NULL;
> > + char errmsg[STRERR_BUFSIZE];
> > + const unsigned long *addrs;
> > + int err, link_fd, prog_fd;
> > + const __u64 *cookies;
> > + const char **syms;
> > + bool retprobe;
> > + size_t cnt;
> > +
> > + if (!OPTS_VALID(opts, bpf_kprobe_multi_opts))
> > + return libbpf_err_ptr(-EINVAL);
> > +
> > + syms = OPTS_GET(opts, syms, false);
> > + addrs = OPTS_GET(opts, addrs, false);
> > + cnt = OPTS_GET(opts, cnt, false);
> > + cookies = OPTS_GET(opts, cookies, false);
> > +
> > + if (!pattern && !addrs && !syms)
> > + return libbpf_err_ptr(-EINVAL);
> > + if (pattern && (addrs || syms || cookies || cnt))
> > + return libbpf_err_ptr(-EINVAL);
> > + if (!pattern && !cnt)
> > + return libbpf_err_ptr(-EINVAL);
> > + if (addrs && syms)
> > + return libbpf_err_ptr(-EINVAL);
> > +
> > + if (pattern) {
> > + err = libbpf_kallsyms_parse(resolve_kprobe_multi_cb, &res);
> > + if (err)
> > + goto error;
> > + if (!res.cnt) {
> > + err = -ENOENT;
> > + goto error;
> > + }
> > + addrs = res.addrs;
> > + cnt = res.cnt;
> > + }
>
> Thanks Jiri.
> Great stuff and a major milestone!
> I've applied Masami's and your patches to bpf-next.

great, thanks

>
> But the above needs more work.
> Currently test_progs -t kprobe_multi
> takes 4 seconds on lockdep+debug kernel.
> Mainly because of the above loop.
>
> 18.05% test_progs [kernel.kallsyms] [k]
> kallsyms_expand_symbol.constprop.4
> 12.53% test_progs libc-2.28.so [.] _IO_vfscanf
> 6.31% test_progs [kernel.kallsyms] [k] number
> 4.66% test_progs [kernel.kallsyms] [k] format_decode
> 4.65% test_progs [kernel.kallsyms] [k] string_nocheck
>
> Single test_skel_api() subtest takes almost a second.

hm, I'll check on lockdep+debug kernel.. I think this test
should be going through kallsyms just once, will check

anyway libbpf_kallsyms_parse could use ksym_get_addr, which is
now cached

>
> A cache inside libbpf probably won't help.
> Maybe introduce a bpf iterator for kallsyms?
>
> On the kernel side kprobe_multi_resolve_syms() looks similarly inefficient.
> I'm not sure whether it would be a bottle neck though.
>
> Orthogonal to this issue please add a new stress test
> to selftest/bpf that attaches to a lot of functions.

ok, will add that

thanks,
jirka