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 an arch_cpu_allowed_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.

Signed-off-by: Will Deacon <will@kernel.org>
---
 kernel/sched/core.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 818c8f7bdf2a..8df38ebfe769 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c

@@ -1696,6 +1696,11 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
 
 #ifdef CONFIG_SMP
 
+/* Must contain at least one active CPU */
+#ifndef arch_cpu_allowed_mask
+#define  arch_cpu_allowed_mask(p)	cpu_possible_mask
+#endif
+
 /*
  * Per-CPU kthreads are allowed to run on !active && online CPUs, see
  * __set_cpus_allowed_ptr() and select_fallback_rq().

@@ -1708,7 +1713,10 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 	if (is_per_cpu_kthread(p))
 		return cpu_online(cpu);
 
-	return cpu_active(cpu);
+	if (!cpu_active(cpu))
+		return false;
+
+	return cpumask_test_cpu(cpu, arch_cpu_allowed_mask(p));
 }
 
 /*

@@ -2361,10 +2369,9 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 			}
 			fallthrough;
 		case possible:
-			do_set_cpus_allowed(p, cpu_possible_mask);
+			do_set_cpus_allowed(p, arch_cpu_allowed_mask(p));