[PATCH v2 0/2] cgroup: expose cpu.stat to BPF

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

As the title suggests, this series adds bpf kfuncs for the cgroup CPU
controller.

Motivation:
Collecting cgroup statistics is expensive: the existing
method is to open and parse a cgroup file. memcg already has an
efficient alternative through BPF; this series extends that idea to cpu.

Design:
- Add bpf_css_flush_rstat() and bpf_cgroup_base_stat() to cgroup rstat.
  The second kfunc returns the raw cgroup_base_stat after the
  same cputime adjustment used by cpu.stat.
- Leave reading the CFS bandwidth counters to the BPF program. They are
  plain fields of tg->cfs_bandwidth, so they need no kernel code.
- Add bpf_css_to_task_group(), which returns a checked RCU pointer of
  the task_group*. This is usefule to compute the throttled time in bpf
  side since the bpf_per_cpu_ptr() requires a verifier-known struct
  task_group * and a pointer carrying the MEM_PERCPU property. 
  This use the convention that task_group embeds its css at offset zero,
  so no scheduler helper or scheduler source change is needed.

===
Changes since v1:
- Make the rstat flush generic, move it to rstat.c, and take a css.
- Return cgroup_base_stat directly instead of repacking its CPU times.
- Replace the throttled-time aggregation kfunc with a checked RCU cast in
  the CPU cgroup BPF code. The cast preserves the verifier type needed for
  the BPF program to perform the per-CPU sum itself.
- Remove the changes in the scheduler part.

Tested on VM with v7.2-rc5. 

Ziyang Men (2):
  cgroup: add BPF kfuncs to read a cpu cgroup's stats
  selftests/bpf: add cgroup_iter_cpu test for cpu cgroup kfuncs

 kernel/cgroup/Makefile                        |   2 +
 kernel/cgroup/bpf_cpu.c                       |  61 +++++
 kernel/cgroup/rstat.c                         |  59 +++-
 tools/testing/selftests/bpf/cgroup_iter_cpu.h |  22 ++
 tools/testing/selftests/bpf/config            |   3 +
 .../bpf/prog_tests/cgroup_iter_cpu.c          | 259 ++++++++++++++++++
 .../selftests/bpf/progs/cgroup_iter_cpu.c     | 113 ++++++++
 7 files changed, 517 insertions(+), 2 deletions(-)
 create mode 100644 kernel/cgroup/bpf_cpu.c
 create mode 100644 tools/testing/selftests/bpf/cgroup_iter_cpu.h
 create mode 100644 tools/testing/selftests/bpf/prog_tests/cgroup_iter_cpu.c
 create mode 100644 tools/testing/selftests/bpf/progs/cgroup_iter_cpu.c

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2.53.0-Meta