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sched: Initialize idle tasks only once

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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
This commit is contained in:
Thomas Gleixner 2024-10-28 11:43:42 +01:00 committed by Peter Zijlstra
parent 1a6151017e
commit b23decf8ac
1 changed files with 5 additions and 7 deletions
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12
kernel/sched/core.c
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@ -4423,7 +4423,8 @@ 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)
{
@ -7697,8 +7698,6 @@ 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);
@ -7713,10 +7712,8 @@ 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
@ -8561,6 +8558,7 @@ 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;