[PATCH v3 0/3] cgroup/cpuset: Keep the top scheduling domain populated

[Guopeng Zhang <guopeng.zhang@linux.dev>]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

Boot-isolated CPUs stay in top_cpuset.effective_cpus, but are excluded
from its scheduling domain. Allocating all boot domain housekeeping CPUs
to partitions can therefore leave the top scheduling domain empty even
though effective_cpus is nonempty, leading to warnings and an OOPS in
build_perf_domains(). CPU hotplug can reach the same state.

The patches are ordered as follows:

  1. Check local and remote partition creation and CPU mask updates.
  2. Allow top-level partition CPU replacements that return CPUs to the
     parent in the same update.
  3. Handle CPU hotplug and preserve partitions that cannot return an
     active boot domain housekeeping CPU.

Changes since v2:
  Keep patch 1's kernel changes unchanged. Update its selftests following
  Longman's and Sashiko's comments and cover recovery with a single
  top-level HK CPU. Add patch 2 to fix the CPU replacement rejection
  reported by Sashiko, with its own regression test. Update the hotplug
  fix, now patch 3, to avoid invalidating unrelated partitions, including
  partitions whose boot housekeeping CPUs are all offline. Drop v2's
  isolated CPU cleanup patch in favor of Ridong's fix:
  https://lore.kernel.org/all/20261107120100.3587595-4-ridong.chen@linux.dev/

  v2: https://lore.kernel.org/all/20261105130344.664336-1-guopeng.zhang@linux.dev/

Changes since v1:
  Rework the fix following Ridong's suggestion and add test cases.
  Add patch 2 for CPU hotplug and patch 3 for isolated CPU cleanup
  when the parent partition becomes invalid.

  v1: https://lore.kernel.org/all/20261104071600.474587-1-guopeng.zhang@linux.dev/

Guopeng Zhang (3):
  cgroup/cpuset: Reject updates that empty the top sched domain
  cgroup/cpuset: Allow top-level partition CPU replacement
  cgroup/cpuset: Keep the top sched domain non-empty on CPU hotplug

 kernel/cgroup/cpuset.c                        | 75 +++++++++++++++++--
 .../selftests/cgroup/test_cpuset_prs.sh       | 68 +++++++++++++++++
 2 files changed, 137 insertions(+), 6 deletions(-)

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2.43.0