Re: [PATCHSET v4 sched_ext/for-7.3] sched_ext: Fix idle CPU state initialization and validation

On Mon Aug 3, 2026 at 6:08 AM UTC, Andrea Righi wrote:
> 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 initializing the built-in idle masks empty. This makes the initial
> state conservative, so busy CPUs are never incorrectly advertised as idle. Once
> the scheduler is enabled and bypass is lifted, the existing idle re-notification
> path reschedules every online CPU. Idle-to-idle re-picks then populate the masks
> with CPUs that are actually idle.
>
> Also rework the allowed_cpus kselftest to replace the racy remote-CPU check with
> a stable local invariant: a CPU running a non-idle scheduling context must not
> be advertised as idle.
>
> Changes in v4:
>  - Initialize the built-in idle masks empty instead of enabling idle tracking
>    before ops.init() and refreshing every CPU under its rq lock (Tejun Heo)
>  - Rely on the existing bypass idle re-notification path to populate the masks
>    with CPUs that are actually idle
>  - Link to v3: https://lore.kernel.org/all/[email protected]/
>
> 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 as busy
>       selftests/sched_ext: Make allowed_cpus idle validation race-free
>
>  kernel/sched/ext/idle.c                            | 16 +++----
>  .../testing/selftests/sched_ext/allowed_cpus.bpf.c | 49 ++++++++++++++++++----
>  2 files changed, 50 insertions(+), 15 deletions(-)

Thanks Andrea, looks good to me!

For the entire series:

Reviewed-by: Kuba Piecuch <[email protected]>