On Fri, Jun 04, 2021 at 06:10:46PM +0100, Valentin Schneider wrote: > 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. That's a good point: even after these patches, we still assume the kernel (and therefore kthreads) can run on all CPUs. I'll expand the comment. > Also, that's an odd place for the definitions, but IIRC there isn't a much > better choice. Short of adding a new header just for this, I couldn't find anything, no. > Reviewed-by: Valentin Schneider <Valentin.Schneider@xxxxxxx> Thanks! Will
