[PATCH v3 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity

[Valentin Schneider]

On 13/11/20 09:37, 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.

{pedantic reading warning}

Hotplugged CPUs *can* be set in a task's affinity mask, though interact
weirdly with cpusets [1]. Having it be the same as hotplug would mean
keeping incompatible CPUs allowed in the affinity mask, but preventing them
from being picked via e.g. is_cpu_allowed().

I was actually hoping this could be a feasible approach, i.e. have an
extra CPU active mask filter for any task:

  cpu_active_mask & arch_cpu_allowed_mask(p)

rather than fiddle with task affinity. Sadly this would also require fixing
up pretty much any place that uses cpu_active_mask(), and probably places
that use p->cpus_ptr as well. RT / DL balancing comes to mind, because that
uses either a task's affinity or a CPU's root domain (which reflects the
cpu_active_mask) to figure out where to push a task.

So while I'm wary of hacking up affinity, I fear it might be the lesser
evil :(

[1]: https://lore.kernel.org/lkml/1251528473.590671.1579196495905.JavaMail.zimbra@efficios.com/

> 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 at kernel.org>