[PATCHSET v3 sched_ext/for-7.3] sched_ext: Fix idle CPU state initialization and validation

The built-in idle masks are initialized with all online CPUs marked idle, but
idle tracking currently starts only after sched_ext is fully enabled. This
leaves busy CPUs incorrectly advertised as idle during ops.init() and until
their next idle transition.

Moreover, the allowed_cpus selftest checks that a remotely selected CPU is no
longer present in the idle mask. An idle-to-idle re-pick can re-advertise the
CPU before the test performs this check, making the validation inherently racy.

Fix this by enabling built-in idle tracking before ops.init() and refreshing the
state of every online CPU under its rq lock. Early tracking updates the built-in
masks, while ops.update_idle() notifications remain suppressed until the
scheduler is fully enabled.

Also rework the allowed_cpus kselftest to replace the racy remote-CPU check with
a stable local invariant that relies on the newly reliable idle-mask state.

Changes in v3:
 - Reuse the built-in idle-selection static key instead of introducing a
   dedicated idle-tracking key (Kuba Piecuch)
 - Check the local CPU-idle invariant from both ops.select_cpu() and
   ops.enqueue() (Kuba Piecuch)
 - Inspect the idle mask without modifying it and perform the check before
   calling scx_bpf_select_cpu_and() (Kuba Piecuch)
 - Link to v2: https://lore.kernel.org/all/[email protected]/

Changes in v2:
 - Move idle-mask initialization from the selftest into the sched_ext
   core (Kuba Piecuch)
 - Add a dedicated idle-tracking static key so transitions are tracked
   before the scheduler is fully enabled (Kuba Piecuch)
 - Rework the allowed_cpus selftest to validate the stable local CPU-idle
   invariant
 - Link to v1: https://lore.kernel.org/all/[email protected]/

Andrea Righi (2):
      sched_ext: Initialize idle masks before ops.init()
      selftests/sched_ext: Make allowed_cpus idle validation race-free

 kernel/sched/ext/ext.h                             |  5 ++-
 kernel/sched/ext/idle.c                            | 35 ++++++++++++++--
 .../testing/selftests/sched_ext/allowed_cpus.bpf.c | 49 ++++++++++++++++++----
 3 files changed, 78 insertions(+), 11 deletions(-)