kthread/smpboot: do not park in kthread_create_on_cpu()

kthread_create_on_cpu() was added by the commit 2a1d446019
("kthread: Implement park/unpark facility").  It is currently used only
when enabling new CPU.  For this purpose, the newly created kthread has to
be parked.

The CPU binding is a bit tricky.  The kthread is parked when the CPU has
not been allowed yet.  And the CPU is bound when the kthread is unparked.

The function would be useful for more per-CPU kthreads, e.g.
bnx2fc_thread, fcoethread.  For this purpose, the newly created kthread
should stay in the uninterruptible state.

This patch moves the parking into smpboot.  It binds the thread already
when created.  Then the function might be used universally.  Also the
behavior is consistent with kthread_create() and kthread_create_on_node().

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Petr Mladek <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Josh Triplett <[email protected]>
Cc: Jiri Kosina <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

kernel/kthread.c

@@ -390,10 +390,10 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
 				   cpu);
 	if (IS_ERR(p))
 		return p;
+	kthread_bind(p, cpu);
+	/* CPU hotplug need to bind once again when unparking the thread. */
 	set_bit(KTHREAD_IS_PER_CPU, &to_kthread(p)->flags);
 	to_kthread(p)->cpu = cpu;
-	/* Park the thread to get it out of TASK_UNINTERRUPTIBLE state */
-	kthread_park(p);
 	return p;
 }
 
@@ -407,6 +407,10 @@ static void __kthread_unpark(struct task_struct *k, struct kthread *kthread)
 	 * which might be about to be cleared.
 	 */
 	if (test_and_clear_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
+		/*
+		 * Newly created kthread was parked when the CPU was offline.
+		 * The binding was lost and we need to set it again.
+		 */
 		if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags))
 			__kthread_bind(k, kthread->cpu, TASK_PARKED);
 		wake_up_state(k, TASK_PARKED);

kernel/smpboot.c

@@ -186,6 +186,11 @@ __smpboot_create_thread(struct smp_hotplug_thread *ht, unsigned int cpu)
 		kfree(td);
 		return PTR_ERR(tsk);
 	}
+	/*
+	 * Park the thread so that it could start right on the CPU
+	 * when it is available.
+	 */
+	kthread_park(tsk);
 	get_task_struct(tsk);
 	*per_cpu_ptr(ht->store, cpu) = tsk;
 	if (ht->create) {