Re: [PATCH v9 12/20] sched: Allow task CPU affinity to be restricted on asymmetric systems

[Valentin Schneider <valentin.schneider@arm.com>]

On 08/06/21 19:03, 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.
>
> Although userspace can carefully manage the affinity masks for such
> tasks, one place where it is particularly problematic is execve()
> because the CPU on which the execve() is occurring may be incompatible
> with the new application image. In such a situation, it is desirable to
> restrict the affinity mask of the task and ensure that the new image is
> entered on a compatible CPU. From userspace's point of view, this looks
> the same as if the incompatible CPUs have been hotplugged off in the
> task's affinity mask. Similarly, if a subsequent execve() reverts to
> a compatible image, then the old affinity is restored if it is still
> valid.
>
> In preparation for restricting the affinity mask for compat tasks on
> arm64 systems without uniform support for 32-bit applications, introduce
> {force,relax}_compatible_cpus_allowed_ptr(), which respectively restrict
> and restore the affinity mask for a task based on the compatible CPUs.
>
> Reviewed-by: Quentin Perret <qperret@google.com>
> Signed-off-by: Will Deacon <will@kernel.org>

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>