[PATCH v6 06/21] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection

[Will Deacon]

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@xxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 include/linux/mmu_context.h |  8 ++++++++
 kernel/sched/core.c         | 10 ++++++----
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/include/linux/mmu_context.h b/include/linux/mmu_context.h
index 03dee12d2b61..bc4ac3c525e6 100644
--- a/include/linux/mmu_context.h
+++ b/include/linux/mmu_context.h
@@ -14,4 +14,12 @@
 static inline void leave_mm(int cpu) { }
 #endif
 
+/*
+ * CPUs that are capable of running task @p. By default, we assume a sane,
+ * homogeneous system. Must contain at least one active CPU.
+ */
+#ifndef task_cpu_possible_mask
+# define task_cpu_possible_mask(p)	cpu_possible_mask
+#endif
+
 #endif
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5226cc26a095..482f7fdca0e8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1813,8 +1813,11 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 		return cpu_online(cpu);
 
 	/* Non kernel threads are not allowed during either online or offline. */
-	if (!(p->flags & PF_KTHREAD))
-		return cpu_active(cpu);
+	if (!(p->flags & PF_KTHREAD)) {
+		if (cpu_active(cpu))
+			return cpumask_test_cpu(cpu, task_cpu_possible_mask(p));
+		return false;
+	}
 
 	/* KTHREAD_IS_PER_CPU is always allowed. */
 	if (kthread_is_per_cpu(p))
@@ -2792,10 +2795,9 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 			 *
 			 * More yuck to audit.
 			 */
-			do_set_cpus_allowed(p, cpu_possible_mask);
+			do_set_cpus_allowed(p, task_cpu_possible_mask(p));
 			state = fail;
 			break;
-
 		case fail:
 			BUG();
 			break;
-- 
2.31.1.751.gd2f1c929bd-goog