Will Deacon 1a96879cb3 FROMLIST: sched: Introduce task_cpu_possible_mask() to limit fallback rq selection ...

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.

Reviewed-by: Quentin Perret <qperret@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Bug: 178507149
Link: https://lore.kernel.org/linux-arch/20201208132835.6151-7-will@kernel.org/
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I75985976c196cee7b84043e1a03fcc62f8b6d1c4

2021-02-05 09:20:52 +00:00

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