[PATCH v4 0/4] selftests: add shared Makefile for BPF selftests and new memcg test

[Ziyang Men <ziyang.meme@gmail.com>]

This series refactors the BPF-related build machinery that is currently
duplicated across selftests/{bpf,sched_ext,hid}/. It adds the shared
tools/testing/selftests/lib.bpf.mk fragment and wires up three consumers,
simplifying both existing BPF selftests and future additions.

The series also adds a cgroup memory-controller selftest that complements
the existing coverage.

Patch 1 adds lib.bpf.mk.

Patch 2 adds selftests/cgroup/test_memcg_stat_cross_cpu, which verifies a
cgroup flush by comparing values read through BPF memcg kfuncs with the
corresponding cgroupfs values. The existing bpf/cgroup_iter_memcg test only
checks whether selected values are non-zero.

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

===
Changes since v3:
For shared Makefile:
- Track in-tree libbpf headers directly so a single make invocation rebuilds
  BPF objects and skeletons after a header change.
- Restore the target definitions needed by LoongArch and MIPS cross builds.
- Validate output directories and skeleton suffixes, and reject dotted or
  hyphenated source stems before they reach build and cleanup rules.
- Add stamp tracking for vmlinux.h generation while preserving content-based
  header updates.
- Make skeletons and subskeletons explicit outputs, so a missing subskeleton is
  regenerated and both sets participate in the all target.
- Namespace private variables, expand the caller-visible contract, track a
  supplied VMLINUX_H, and remove unnecessary include search paths.

For cgroup selftest:
- Add CONFIG_BPF_EVENTS and probe every required kfunc plus the sleepable
  cgroup iterator before running.
- Bound CPU fanout, cgroup/process count, and total memory use; skip a case
  when the flush threshold would require more than 256 MiB.
- Check CPU discovery, polling, iterator creation, and BPF errors, with clearer
  diagnostics.
- Compare each BPF statistic against two following memory.stat samples, keeping
  exact equality on a quiet subtree while tolerating values that move between
  the two samples.

For hid and sched_ext:
- Remove unused HID locals instead of suppressing warnings, drop obsolete
  variables and comments, tighten cleanup, and correct the generated-output
  description.

===
Changes since v2:
For shared Makefile:
- Use USERCFLAGS and USERLDFLAGS to extend CFLAGS and LDFLAGS.
- Pass OPT_FLAGS/RELEASE and EXTRA_CFLAGS to the libbpf and bpftool sub-makes.
- Use one rule per BPF source instead of a global vpath that also affected the
  caller's normal %.c rules.
- Take HOSTCC/HOSTLD from lib.mk instead of including Makefile.include, which
  also rewrote the caller's AR, LD, and CFLAGS.
- Report a missing vmlinux from the recipe rather than at parse time, so
  "make clean" still works without BTF.

For cgroup selftest:
- Create one cgroup tree, perform one flush, then read through BPF and
  cgroupfs.
- Use common cgroup helpers such as cg_read_key_long() and values_close().
- Use cpu_set_t rather than manually allocated CPU lists.
- Move test_memcontrol.c's anonymous-memory allocator into cgroup_util.
- Address the remaining review feedback.

===
Changes since v1:
- Generalize lib.bpf.mk for source layouts and suffixes, extra headers and
  flags, skeleton suffixes, subskeletons, and configurable output directories.
- Add patch 3 (hid) and patch 4 (sched_ext), converting those directories 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       |  46 ++
 tools/testing/selftests/cgroup/config         |   5 +
 .../selftests/cgroup/lib/cgroup_util.c        |  82 ++
 .../cgroup/lib/include/cgroup_util.h          |   3 +
 .../cgroup/memcg_stat_cross_cpu.bpf.c         |  95 +++
 .../selftests/cgroup/memcg_stat_cross_cpu.h   |  18 +
 .../cgroup/test_memcg_stat_cross_cpu.c        | 755 ++++++++++++++++++
 .../selftests/cgroup/test_memcontrol.c        |  29 +-
 tools/testing/selftests/hid/.gitignore        |   1 +
 tools/testing/selftests/hid/Makefile          | 179 +----
 tools/testing/selftests/hid/progs/hid.c       |   7 +-
 tools/testing/selftests/lib.bpf.mk            | 321 ++++++++
 tools/testing/selftests/sched_ext/Makefile    | 149 +---
 14 files changed, 1382 insertions(+), 316 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