Re: [PATCH v4 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity

[Quentin Perret <qperret@google.com>]

On Tuesday 24 Nov 2020 at 15:50:32 (+0000), 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 its
> affinity mask.
> 
> In preparation for restricting the affinity mask for compat tasks on
> arm64 systems without uniform support for 32-bit applications, introduce
> a restrict_cpus_allowed_ptr(), which allows the current affinity mask
> for a task to be shrunk to the intersection of a parameter mask.
> 
> Signed-off-by: Will Deacon <will@kernel.org>

Reviewed-by: Quentin Perret <qperret@google.com>

Thanks,
Quentin