stop_machine: Avoid a sleep and wakeup in stop_one_cpu()

In case @cpu == smp_proccessor_id(), we can avoid a sleep+wakeup
cycle by doing a preemption.

Callers such as sched_exec() can benefit from this change.
Signed-off-by: Cheng Chao <cs.os.kernel@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: akpm@linux-foundation.org
Cc: chris@chris-wilson.co.uk
Cc: tj@kernel.org
Link: http://lkml.kernel.org/r/1473818510-6779-1-git-send-email-cs.os.kernel@gmail.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -1063,8 +1063,12 @@ static int migration_cpu_stop(void *data)
 	 * holding rq->lock, if p->on_rq == 0 it cannot get enqueued because
 	 * we're holding p->pi_lock.
 	 */
-	if (task_rq(p) == rq && task_on_rq_queued(p))
+	if (task_rq(p) == rq) {
+		if (task_on_rq_queued(p))
 			rq = __migrate_task(rq, p, arg->dest_cpu);
+		else
+			p->wake_cpu = arg->dest_cpu;
+	}
 	raw_spin_unlock(&rq->lock);
 	raw_spin_unlock(&p->pi_lock);
 

kernel/stop_machine.c

@@ -126,6 +126,11 @@ 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;
+	/*
+	 * In case @cpu == smp_proccessor_id() we can avoid a sleep+wakeup
+	 * cycle by doing a preemption:
+	 */
+	cond_resched();
 	wait_for_completion(&done.completion);
 	return done.ret;
 }