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    SubjectRe: [PATCH v3 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity
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    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@kernel.org>

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