[PATCH v2] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT_NONE

[Cheng Chao]

For CONFIG_PREEMPT_NONE=y, when sched_exec() needs migration, sched_exec()
calls stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg).

If the migration_cpu_stop() can not migrate,why do we call stop_one_cpu()?
It just makes the task TASK_UNINTERRUPTIBLE, wakes up the stopper thread,
executes migration_cpu_stop(), and the stopper thread wakes up the task.

But in fact, all above works are almost useless(wasteful),the reason is 
migration_cpu_stop() can not migrate. why? migration_cpu_stop() needs the 
task is TASK_ON_RQ_QUEUED before it calls __migrate_task().

This patch keeps the task TASK_RUNNING instead of TASK_UNINTERRUPTIBLE,
so the migration_cpu_stop() can do useful works.

Signed-off-by: Cheng Chao <chengchao@xxxxxxxxxxx>
---
 kernel/stop_machine.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 4a1ca5f..41aea5e 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -126,6 +126,17 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
 	cpu_stop_init_done(&done, 1);
 	if (!cpu_stop_queue_work(cpu, &work))
 		return -ENOENT;
+
+#if defined(CONFIG_PREEMPT_NONE)
+	/*
+	 * Makes the stopper thread run as soon as possible.
+	 * And if the caller is TASK_RUNNING, keeps the caller TASK_RUNNING.
+	 * It's special useful for some callers which are expected to be
+	 * TASK_ON_RQ_QUEUED.
+	 * sched_exec does benefit from this improvement.
+	 */
+	schedule();
+#endif
 	wait_for_completion(&done.completion);
 	return done.ret;
 }
-- 
2.4.11