sched,init: Fix DEBUG_PREEMPT vs early boot

Extend 8fb12156 ("init: Pin init task to the boot CPU, initially")
to cover the new PF_NO_SETAFFINITY requirement.

While there, move wait_for_completion(&kthreadd_done) into kernel_init()
to make it absolutely clear it is the very first thing done by the init
thread.

Fixes: 570a752b ("lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Borislav Petkov <bp@alien8.de>
Link: https://lkml.kernel.org/r/YLS4mbKUrA3Gnb4t@hirez.programming.kicks-ass.net

init/main.c

@@ -692,6 +692,7 @@ noinline void __ref rest_init(void)
 	 */
 	rcu_read_lock();
 	tsk = find_task_by_pid_ns(pid, &init_pid_ns);
+	tsk->flags |= PF_NO_SETAFFINITY;
 	set_cpus_allowed_ptr(tsk, cpumask_of(smp_processor_id()));
 	rcu_read_unlock();
 
@@ -1440,6 +1441,11 @@ static int __ref kernel_init(void *unused)
 {
 	int ret;
 
+	/*
+	 * Wait until kthreadd is all set-up.
+	 */
+	wait_for_completion(&kthreadd_done);
+
 	kernel_init_freeable();
 	/* need to finish all async __init code before freeing the memory */
 	async_synchronize_full();
@@ -1520,11 +1526,6 @@ void __init console_on_rootfs(void)
 
 static noinline void __init kernel_init_freeable(void)
 {
-	/*
-	 * Wait until kthreadd is all set-up.
-	 */
-	wait_for_completion(&kthreadd_done);
-
 	/* Now the scheduler is fully set up and can do blocking allocations */
 	gfp_allowed_mask = __GFP_BITS_MASK;
 

kernel/sched/core.c

@@ -8862,6 +8862,7 @@ void __init sched_init_smp(void)
 	/* Move init over to a non-isolated CPU */
 	if (set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0)
 		BUG();
+	current->flags &= ~PF_NO_SETAFFINITY;
 	sched_init_granularity();
 
 	init_sched_rt_class();