[PATCH v5 06/15] 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 at google.com>
Signed-off-by: Will Deacon <will at kernel.org>
---
 include/linux/mmu_context.h | 8 ++++++++
 kernel/sched/core.c         | 8 +++++---
 2 files changed, 13 insertions(+), 3 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 e7e453492cff..58474569a2ea 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1708,7 +1708,10 @@ 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, task_cpu_possible_mask(p));
 }
 
 /*
@@ -2318,10 +2321,9 @@ 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, task_cpu_possible_mask(p));
 			state = fail;
 			break;
-
 		case fail:
 			BUG();
 			break;
-- 
2.29.2.576.ga3fc446d84-goog