sched: Initialize idle tasks only once

[ Upstream commit b23decf ]

Idle tasks are initialized via __sched_fork() twice:

     fork_idle()
        copy_process()
	  sched_fork()
             __sched_fork()
	init_idle()
          __sched_fork()

Instead of cleaning this up, sched_ext hacked around it. Even when analyis
and solution were provided in a discussion, nobody cared to clean this up.

init_idle() is also invoked from sched_init() to initialize the boot CPU's
idle task, which requires the __sched_fork() invocation. But this can be
trivially solved by invoking __sched_fork() before init_idle() in
sched_init() and removing the __sched_fork() invocation from init_idle().

Do so and clean up the comments explaining this historical leftover.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241028103142.359584747@linutronix.de
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: KAGA-KOKO

kernel/sched/core.c

@@ -4488,7 +4488,8 @@ int wake_up_state(struct task_struct *p, unsigned int state)
  * Perform scheduler related setup for a newly forked process p.
  * p is forked by current.
  *
- * __sched_fork() is basic setup used by init_idle() too:
+ * __sched_fork() is basic setup which is also used by sched_init() to
+ * initialize the boot CPU's idle task.
  */
 static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 {
@@ -9257,8 +9258,6 @@ void __init init_idle(struct task_struct *idle, int cpu)
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long flags;
 
-	__sched_fork(0, idle);
-
 	raw_spin_lock_irqsave(&idle->pi_lock, flags);
 	raw_spin_rq_lock(rq);
 
@@ -9273,10 +9272,8 @@ void __init init_idle(struct task_struct *idle, int cpu)
 
 #ifdef CONFIG_SMP
 	/*
-	 * It's possible that init_idle() gets called multiple times on a task,
-	 * in that case do_set_cpus_allowed() will not do the right thing.
-	 *
-	 * And since this is boot we can forgo the serialization.
+	 * No validation and serialization required at boot time and for
+	 * setting up the idle tasks of not yet online CPUs.
 	 */
 	set_cpus_allowed_common(idle, &ac);
 #endif
@@ -10105,6 +10102,7 @@ void __init sched_init(void)
 	 * but because we are the idle thread, we just pick up running again
 	 * when this runqueue becomes "idle".
 	 */
+	__sched_fork(0, current);
 	init_idle(current, smp_processor_id());
 
 	calc_load_update = jiffies + LOAD_FREQ;