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SubjectRe: [PATCH v8 05/19] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection
On 02/06/21 17:47, Will Deacon wrote:
> Asymmetric systems may not offer the same level of userspace ISA support
> across all CPUs, meaning that some applications cannot be executed by
> some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do
> not feature support for 32-bit applications on both clusters.
> On such a system, we must take care not to migrate a task to an
> unsupported CPU when forcefully moving tasks in select_fallback_rq()
> in response to a CPU hot-unplug operation.
> Introduce a task_cpu_possible_mask() hook which, given a task argument,
> allows an architecture to return a cpumask of CPUs that are capable of
> executing that task. The default implementation returns the
> cpu_possible_mask, since sane machines do not suffer from per-cpu ISA
> limitations that affect scheduling. The new mask is used when selecting
> the fallback runqueue as a last resort before forcing a migration to the
> first active CPU.

Nit: Some uses of this mask (cpu_is_allowed(), __set_cpus_allowed_ptr())
don't apply to kthreads. This makes sense for the 32-bit@EL0 faff, but it
wouldn't hurt to point this out somewhere IMO.

Also, that's an odd place for the definitions, but IIRC there isn't a much
better choice.

Reviewed-by: Valentin Schneider <>

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