[PATCH 3/5] sched_exec() should use select_fallback_rq() logic

sched_exec()->select_task_rq() reads/updates ->cpus_allowed lockless.
This can race with other CPUs updating our ->cpus_allowed, and this
looks meaningless to me.

The task is current and running, it must have online cpus in ->cpus_allowed,
the fallback mode is bogus. And, if ->sched_class returns the "wrong" cpu,
this likely means we raced with set_cpus_allowed() which was called
for reason, why should sched_exec() retry and call ->select_task_rq()
again?

Change the code to call sched_class->select_task_rq() directly and do
nothing if the returned cpu is wrong after re-checking under rq->lock.

>From now select_fallback_rq() is always called with either rq-lock or
TASK_WAKING held.

TODO: update the comments.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---


--- 34-rc1/kernel/sched.c~3_SCHED_EXEC_DONT_FALLBACK	2010-03-13 19:50:57.000000000 +0100
+++ 34-rc1/kernel/sched.c	2010-03-13 19:51:11.000000000 +0100
@@ -3123,9 +3123,8 @@ void sched_exec(void)
 	unsigned long flags;
 	struct rq *rq;
 
-again:
 	this_cpu = get_cpu();
-	dest_cpu = select_task_rq(p, SD_BALANCE_EXEC, 0);
+	dest_cpu = p->sched_class->select_task_rq(p, SD_BALANCE_EXEC, 0);
 	if (dest_cpu == this_cpu) {
 		put_cpu();
 		return;
@@ -3133,18 +3132,12 @@ again:
 
 	rq = task_rq_lock(p, &flags);
 	put_cpu();
-
 	/*
 	 * select_task_rq() can race against ->cpus_allowed
 	 */
-	if (!cpumask_test_cpu(dest_cpu, &p->cpus_allowed)
-	    || unlikely(!cpu_active(dest_cpu))) {
-		task_rq_unlock(rq, &flags);
-		goto again;
-	}
-
-	/* force the process onto the specified CPU */
-	if (migrate_task(p, dest_cpu, &req)) {
+	if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed) &&
+	    likely(cpu_active(dest_cpu)) &&
+	    migrate_task(p, dest_cpu, &req)) {
 		/* Need to wait for migration thread (might exit: take ref). */
 		struct task_struct *mt = rq->migration_thread;