[PATCH] sched: Fix migration_cpu_stop() return value

[Peter Zijlstra <peterz@infradead.org>]

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A while ago I did a similar patch for some debugging, but looking at it
again today I realized we should probably fix this anyway.

---
Subject: sched: Fix migration_cpu_stop() return value

There are a number of migration_cpu_stop() users; and some actually care
about the success of the migration. So report this.

In particular migrate_task_to() as used from task_numa_migrate()
actually tests this return value.

Also change set_cpus_allowed_ptr() to propagate this return value, since
it already returns other errors.

Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
---
 kernel/sched/core.c |   15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4633,11 +4633,13 @@ int set_cpus_allowed_ptr(struct task_str
 	dest_cpu = cpumask_any_and(cpu_active_mask, new_mask);
 	if (p->on_rq) {
 		struct migration_arg arg = { p, dest_cpu };
+
 		/* Need help from migration thread: drop lock and wait. */
 		task_rq_unlock(rq, p, &flags);
-		stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
+		ret = stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
 		tlb_migrate_finish(p->mm);
-		return 0;
+
+		return ret;
 	}
 out:
 	task_rq_unlock(rq, p, &flags);
@@ -4747,19 +4749,24 @@ void sched_setnuma(struct task_struct *p
  * migration_cpu_stop - this will be executed by a highprio stopper thread
  * and performs thread migration by bumping thread off CPU then
  * 'pushing' onto another runqueue.
+ *
+ * Returns 0 on success, -EAGAIN on failure to migrate.
  */
 static int migration_cpu_stop(void *data)
 {
 	struct migration_arg *arg = data;
+	int ret = 0;
 
 	/*
 	 * The original target cpu might have gone down and we might
 	 * be on another cpu but it doesn't matter.
 	 */
 	local_irq_disable();
-	__migrate_task(arg->task, raw_smp_processor_id(), arg->dest_cpu);
+	if (!__migrate_task(arg->task, raw_smp_processor_id(), arg->dest_cpu))
+		ret = -EAGAIN;
 	local_irq_enable();
-	return 0;
+
+	return ret;
 }
 
 #ifdef CONFIG_HOTPLUG_CPU

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