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

From: Cheng Chao
Date: Fri Sep 09 2016 - 04:14:32 EST

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;
+ /*
+ * 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
+ * sched_exec does benefit from this improvement.
+ */
+ schedule();
return done.ret;