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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/20260731090334.2911948-1-arighi@nvidia.com/ 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/20260726064754.378671-1-arighi@nvidia.com/ 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(-)