Patch "sched: Initialize idle tasks only once" has been added to the 6.6-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    sched: Initialize idle tasks only once

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     sched-initialize-idle-tasks-only-once.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 1c68684e10d7b59244de0957982a8abfcc6fa994
Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Date:   Mon Oct 28 11:43:42 2024 +0100

    sched: Initialize idle tasks only once
    
    [ Upstream commit b23decf8ac9102fc52c4de5196f4dc0a5f3eb80b ]
    
    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@xxxxxxxxxxxxx>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
    Link: https://lore.kernel.org/r/20241028103142.359584747@xxxxxxxxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 228f7c07da72..86606fb9e6bc 100644
--- a/kernel/sched/core.c
+++ b/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;