[tip: sched/urgent] kernel/sched: Modify initial boot task idle setup

[tip-bot2 for Liam R. Howlett]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     cff9b2332ab762b7e0586c793c431a8f2ea4db04
Gitweb:        https://git.kernel.org/tip/cff9b2332ab762b7e0586c793c431a8f2ea4db04
Author:        Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
AuthorDate:    Fri, 15 Sep 2023 13:44:44 -04:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 19 Sep 2023 10:48:04 +02:00

kernel/sched: Modify initial boot task idle setup

Initial booting is setting the task flag to idle (PF_IDLE) by the call
path sched_init() -> init_idle().  Having the task idle and calling
call_rcu() in kernel/rcu/tiny.c means that TIF_NEED_RESCHED will be
set.  Subsequent calls to any cond_resched() will enable IRQs,
potentially earlier than the IRQ setup has completed.  Recent changes
have caused just this scenario and IRQs have been enabled early.

This causes a warning later in start_kernel() as interrupts are enabled
before they are fully set up.

Fix this issue by setting the PF_IDLE flag later in the boot sequence.

Although the boot task was marked as idle since (at least) d80e4fda576d,
I am not sure that it is wrong to do so.  The forced context-switch on
idle task was introduced in the tiny_rcu update, so I'm going to claim
this fixes 5f6130fa52ee.

Fixes: 5f6130fa52ee ("tiny_rcu: Directly force QS when call_rcu_[bh|sched]() on idle_task")
Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/linux-mm/CAMuHMdWpvpWoDa=Ox-do92czYRvkok6_x6pYUH+ZouMcJbXy+Q@xxxxxxxxxxxxxx/
---
 kernel/sched/core.c | 2 +-
 kernel/sched/idle.c | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2299a5c..802551e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9269,7 +9269,7 @@ void __init init_idle(struct task_struct *idle, int cpu)
 	 * PF_KTHREAD should already be set at this point; regardless, make it
 	 * look like a proper per-CPU kthread.
 	 */
-	idle->flags |= PF_IDLE | PF_KTHREAD | PF_NO_SETAFFINITY;
+	idle->flags |= PF_KTHREAD | PF_NO_SETAFFINITY;
 	kthread_set_per_cpu(idle, cpu);
 
 #ifdef CONFIG_SMP
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 342f58a..5007b25 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -373,6 +373,7 @@ EXPORT_SYMBOL_GPL(play_idle_precise);
 
 void cpu_startup_entry(enum cpuhp_state state)
 {
+	current->flags |= PF_IDLE;
 	arch_cpu_idle_prepare();
 	cpuhp_online_idle(state);
 	while (1)