Re: 'perf test BPF' failing, libbpf regression wrt "basic API for BPF obj name"

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Wed, Nov 29, 2017 at 02:31:36PM -0800, Martin KaFai Lau escreveu:
> On Wed, Nov 29, 2017 at 06:15:43PM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Wed, Nov 29, 2017 at 01:07:34PM -0800, Martin KaFai Lau escreveu:
> > > On Tue, Nov 28, 2017 at 04:05:19PM -0300, Arnaldo Carvalho de Melo wrote:
> > > > [root@jouet ~]# perf test -v bpf
> > > > 39: BPF filter                                            :
> > > > 39.1: Basic BPF filtering                                 :
> > > > --- start ---
> > > > test child forked, pid 12198
> > > > Kernel build dir is set to /lib/modules/4.14.0+/build
> > > > set env: KBUILD_DIR=/lib/modules/4.14.0+/build
> > > [ ... ]
> > > > libbpf: failed to create map (name: 'flip_table'): Invalid argument
> > > > libbpf: failed to load object '[basic_bpf_test]'
> > > > bpf: load objects failed
> > > 88cda1c9da02 ("bpf: libbpf: Provide basic API support to specify BPF obj name")
> > > is introduced in 4.15.

> > > I think the perf@kernel-4.15 broke on older kernels like 4.14 because
> > > the new bpf prog/map name is only introduced since 4.15.

> > > The newer perf needs to be compatible with an older kernel?

> > Sure :-)

> Would the latest features introduced in perf/libbpf supposed to be
> available in the latest kernel only?  What may be the reason that the

Yes, then the new perf binary should try to use the new stuff, if it
fails, use the old one, there is no requirement that one uses perf 4.14
in lockstep with the kernel 4.14 (or any other version), perf 4.15
should work with the 4.14 kernel as well as with 4.15 (or any other
future kernel), only limited by what it can grok up to when it was
released.

I'll test with a 4.15 kernel to confirm this, thanks for the
explanation!

> latest perf is installed with an older kernel while it does not gain new
> functionality? 

See, for instance tools/perf/util/evsel.c, function perf_evsel__open(),
and all those fallbacks for features a new tool doesn't find on an older
kernel.

When the user asks for some feature that is not present in the older
kernel, then it fails and the tool calling perf_evsel__open() will use
perf_evlist__strerror_open() to provide as best an explanation to what
took place as possible.

- Arnaldo