[PATCH v3 0/4] selftests: shared lib.bpf.mk for building BPF progs and skeletons

This series re-factors the bpf-related toolchain for selftests, which
currently duplicated across selftests/{bpf,sched_ext,hid}/. Unify them
into a single includable fragment, the tools/testing/selftests/lib.bpf.mk,
and wires up three consumers. We believe this will simplify the work for
configuring selftests with the bpf, for both the existing and future ones.

Patch 1 adds lib.bpf.mk mentioned above.

Patch 2 adds a new selftest, selftests/cgroup/test_memcg_stat_cross_cpu,
which checks the cgroup flush happens correctly by comparing the bpf
read value matches the file reading. Existing test
bpf/cgroup_iter_memcg only ensures whether the number is non-zero, which
is trivial in some cases.

Patches 3 and 4 replace the duplicated parts in selftests/{hid,
sched_ext} with the lib.bpf.mk

===
Changes since v2:
For shared Makefile:
- The lib.bpf.mk now use the USERCFLAGS (and USERLDFLAGS) to extend the
  CFLAGS and LDFLAGS. 
- OPT_FLAGS/RELEASE and EXTRA_CFLAGS now reach the libbpf and bpftool
  sub-makes as well.
- One rule per BPF source instead of a vpath, which is global and also
  applied to the caller's own %.c rules.
- HOSTCC/HOSTLD come from lib.mk instead of including Makefile.include,
  which also rewrote the caller's AR, LD and CFLAGS.
- A missing vmlinux is reported by the recipe rather than at parse time,
  so "make clean" still works without BTF.

For cgroup selftest:
- Only one cgroup tree is created, one flush is performed, follows by
  bpf read and file read. 
- More common cgroup functions e.g., cg_read_key_long() and
  value_close(), are used in the selftests.
- Use cpu_set_t rather than manually allocated cpu lists.
- Use test_memcontrol.c's anon allocator and moving it into cgroup_util.
- Other suggestion from the review.

Others:
- Bot reviews are fixed.

===
Changes since v1:
  - Generalized lib.bpf.mk (source layout/suffix, extra hdrs/cflags,
    skeleton suffix, subskeletons, configurable output dirs) so it can
    serve hid and sched_ext, not just cgroup.
  - Added patch 3 (hid) and patch 4 (sched_ext), converting those folders
    to the shared fragment.

Ziyang Men (4):
  selftests: add shared lib.bpf.mk to build BPF progs and skeletons
  selftests/cgroup: add memcg_stat_cross_cpu correctness test for flush
  selftests/hid: build the BPF program via the shared lib.bpf.mk
  selftests/sched_ext: build BPF schedulers via the shared lib.bpf.mk

 tools/testing/selftests/cgroup/.gitignore     |   8 +
 tools/testing/selftests/cgroup/Makefile       |  52 ++
 tools/testing/selftests/cgroup/config         |   5 +
 .../selftests/cgroup/lib/cgroup_util.c        |  83 +++
 .../cgroup/lib/include/cgroup_util.h          |   3 +
 .../cgroup/memcg_stat_cross_cpu.bpf.c         |  86 +++
 .../selftests/cgroup/memcg_stat_cross_cpu.h   |  21 +
 .../cgroup/test_memcg_stat_cross_cpu.c        | 596 ++++++++++++++++++
 .../selftests/cgroup/test_memcontrol.c        |  29 +-
 tools/testing/selftests/hid/.gitignore        |   1 +
 tools/testing/selftests/hid/Makefile          | 180 +-----
 tools/testing/selftests/lib.bpf.mk            | 296 +++++++++
 tools/testing/selftests/sched_ext/Makefile    | 146 +----
 13 files changed, 1204 insertions(+), 302 deletions(-)
 create mode 100644 tools/testing/selftests/cgroup/memcg_stat_cross_cpu.bpf.c
 create mode 100644 tools/testing/selftests/cgroup/memcg_stat_cross_cpu.h
 create mode 100644 tools/testing/selftests/cgroup/test_memcg_stat_cross_cpu.c
 create mode 100644 tools/testing/selftests/lib.bpf.mk

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2.53.0-Meta