[PATCHSET sched_ext/for-7.3] sched_ext: Cgroup migration and op delivery for sub-schedulers

[Tejun Heo]

Hello,

A task's sched must match its cgroup's: each sub-scheduler serves the
cgroup2 subtree it is attached to and the root scheduler serves the rest.
Cgroup migration currently breaks this. A task moved across a
sub-scheduler boundary keeps its old sched, leading to wrong-sched
scheduling and, once the stale sched is freed, a use-after-free. Cgroup
ops are also always delivered to the root scheduler no matter which sched
serves the cgroup.

This patchset makes both follow the sub-scheduler topology:

- cgroup gains a task migration notifier (0001). sched_ext uses it to
  re-home tasks whose migration crosses a sched boundary: the destination
  sched runs the fallible ops.init_task() before the migration commits,
  so a rejection fails the cgroup.procs write (0002-0004).

- cgroup_init/exit and the knob ops are delivered to the sched each
  task_group is on. The move ops go to the task's sched, only for
  migrations that don't re-home it (0005).

- Sub-scheduler enable claims the subtree's cgroups from the parent and
  disable returns them. A parent that fails to re-init a returned cgroup
  fails in turn and the cgroups move up the chain, ending at the root
  scheduler (0006).

- scx_qmap consumes cgroup weights through ops.cgroup_set_weight() and
  gains fault injection modes to exercise the failure paths (0007-0008).

Verified with a trace-based suite on the demo scheduler covering op
delivery and routing, handover failures, multi-level punt chains, cgroup1
and stress.

Based on sched_ext/for-7.3 (7c2cd767705d).

Tejun Heo (8):
  cgroup: Add cgroup_task_notifier and task migration events
  sched_ext: Factor out scx_rehome_task() and scx_punt_task()
  sched_ext: Relocate scx_cgroup_enabled
  sched_ext: Re-home tasks on cgroup migration
  sched_ext: Deliver cgroup ops to each task_group's sched
  sched_ext: Hand over cgroups at sub-scheduler enable/disable
  tools/sched_ext: scx_qmap - Consume cgroup weights through set_weight
  tools/sched_ext: scx_qmap - Add init fault injection modes

Git tree: git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git scx-cgroup-migration

 include/linux/cgroup.h         |  26 +++
 include/linux/sched/ext.h      |  15 ++
 kernel/cgroup/cgroup.c         |  93 ++++++++-
 kernel/sched/ext/ext.c         | 247 +++++++++++++++++-------
 kernel/sched/ext/internal.h    |  44 ++++-
 kernel/sched/ext/sub.c         | 418 +++++++++++++++++++++++++++++++++++++----
 tools/sched_ext/scx_qmap.bpf.c |  95 ++++++++--
 tools/sched_ext/scx_qmap.c     | 110 +----------
 tools/sched_ext/scx_qmap.h     |   5 +-
 9 files changed, 819 insertions(+), 234 deletions(-)

--
tejun