sched: Migrate waking tasks

Current code can fail to migrate a waking task (silently) when TTWU_QUEUE is
enabled.

When a task is waking, it is pending on the wake_list of the rq, but it is not
queued (task->on_rq == 0). In this case, set_cpus_allowed_ptr() and
__migrate_task() will not migrate it because its invisible to them.

This behavior is incorrect, because the task has been already woken, it will be
running on the wrong CPU without correct placement until the next wake-up or
update for cpus_allowed.

To fix this problem, we need to finish the wakeup (so they appear on
the runqueue) before we migrate them.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Reported-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>
Tested-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/538ED7EB.5050303@cn.fujitsu.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -4666,7 +4666,7 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
 		goto out;
 
 	dest_cpu = cpumask_any_and(cpu_active_mask, new_mask);
-	if (task_on_rq_queued(p)) {
+	if (task_on_rq_queued(p) || p->state == TASK_WAKING) {
 		struct migration_arg arg = { p, dest_cpu };
 		/* Need help from migration thread: drop lock and wait. */
 		task_rq_unlock(rq, p, &flags);
@@ -4799,6 +4799,12 @@ static int migration_cpu_stop(void *data)
 	 * be on another cpu but it doesn't matter.
 	 */
 	local_irq_disable();
+	/*
+	 * We need to explicitly wake pending tasks before running
+	 * __migrate_task() such that we will not miss enforcing cpus_allowed
+	 * during wakeups, see set_cpus_allowed_ptr()'s TASK_WAKING test.
+	 */
+	sched_ttwu_pending();
 	__migrate_task(arg->task, raw_smp_processor_id(), arg->dest_cpu);
 	local_irq_enable();
 	return 0;