[PATCH v5 bpf-next 0/4] bpf: Add bpf_iter_cpumask

[Yafang Shao <laoar.shao@gmail.com>]

Three new kfuncs, namely bpf_iter_cpumask_{new,next,destroy}, have been
added for the new bpf_iter_cpumask functionality. These kfuncs enable the
iteration of percpu data, such as runqueues, system_group_pcpu, and more.

In our specific use case, we leverage the cgroup iterator to traverse
percpu data, subsequently exposing it to userspace through a seq file.
Refer to example in patch #2 for the usage.

Changes:
- v4 -> v5:
  - Use cpumask_size() instead of sizeof(struct cpumask) (David)
  - Refactor the selftests as suggsted by David
  - Improve the doc as suggested by David
- v3 -> v4:
  - Use a copy of cpumask to avoid potential use-after-free (Yonghong)
  - Various code improvement in selftests (Yonghong)
- v2 -> v3:
  - Define KF_RCU_PROTECTED for bpf_iter_cpumask_new (Alexei)
  - Code improvement in selftests
  - Fix build error in selftest due to CONFIG_PSI=n
    reported by kernel test robot <lkp@intel.com>
- v1 -> v2: 
  - Avoid changing cgroup subsystem (Tejun)
  - Remove bpf_cpumask_set_from_pid(), and use bpf_cpumask_copy()
    instead (Tejun)
  - Use `int cpu;` field in bpf_iter_cpumask_kern (Andrii)
- bpf: Add new bpf helper bpf_for_each_cpu
  https://lwn.net/ml/bpf/20230801142912.55078-1-laoar.shao@gmail.com/

Yafang Shao (4):
  bpf: Add bpf_iter_cpumask kfuncs
  bpf, docs: Add document for cpumask iter
  selftests/bpf: Fix error checking for cpumask_success__load()
  selftests/bpf: Add selftests for cpumask iter

 Documentation/bpf/cpumasks.rst                |  60 +++++++
 kernel/bpf/cpumask.c                          |  82 +++++++++
 tools/testing/selftests/bpf/config            |   1 +
 .../selftests/bpf/prog_tests/cpumask.c        | 158 +++++++++++++++++-
 .../selftests/bpf/progs/cpumask_common.h      |   3 +
 .../bpf/progs/cpumask_iter_failure.c          |  99 +++++++++++
 .../bpf/progs/cpumask_iter_success.c          | 126 ++++++++++++++
 7 files changed, 526 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/cpumask_iter_failure.c
 create mode 100644 tools/testing/selftests/bpf/progs/cpumask_iter_success.c

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2.39.1