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 <Valentin.Schneider@xxxxxxx>
