Re: ~90s reboot delay with v6.19 and PREEMPT_RT

[Bert Karwatzki <spasswolf@web.de>]

Am Mittwoch, dem 25.02.2026 um 16:43 +0100 schrieb Sebastian Andrzej Siewior:
> On 2026-02-19 17:46:47 [+0100], Bert Karwatzki wrote:
> > Since linux v6.19 I noticed that rebooting my MSI Alpha 15 Laptop
> > would hang for about ~90s before rebooting. I bisected this (from
> > v6.18 to v6.19) and got this as the first bad commit:
> > 9311e6c29b34 ("cgroup: Fix sleeping from invalid context warning on PREEMPT_RT")
> …
> 
> I'm on it. I looks like we free the task after sched_process_wait() but
> before it is entirely gone there is a wait() on its pid. Some of them do
> come back but one seems to be stuck and I need to figure out which one.
> If we get rid of the LAZY then it happens "quick" enough so it works.
> 
> > Bert Karwatzki
> 
> Sebastian

I've done two testruns with this debug patch (The persistant log buffer works now, thanks
again to Steven Rostedt):

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 5f0d33b04910..b750aa284b89 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -6990,6 +6990,7 @@ static void do_cgroup_task_dead(struct task_struct *tsk)
 {
 	struct css_set *cset;
 	unsigned long flags;
+	trace_printk(KERN_INFO "%s 0: task = %px\n", __func__, tsk);
 
 	spin_lock_irqsave(&css_set_lock, flags);
 
@@ -7029,9 +7030,11 @@ static void cgrp_dead_tasks_iwork_fn(struct irq_work *iwork)
 {
 	struct llist_node *lnode;
 	struct task_struct *task, *next;
+	trace_printk(KERN_INFO "%s:\n", __func__);
 
 	lnode = llist_del_all(this_cpu_ptr(&cgrp_dead_tasks));
 	llist_for_each_entry_safe(task, next, lnode, cg_dead_lnode) {
+		trace_printk(KERN_INFO "%s: %px %s", __func__, task, task->comm);
 		do_cgroup_task_dead(task);
 		put_task_struct(task);
 	}
@@ -7050,6 +7053,7 @@ static void __init cgroup_rt_init(void)
 
 void cgroup_task_dead(struct task_struct *task)
 {
+	trace_printk(KERN_INFO "%s: task = %px (%s)\n", __func__, task, task->comm);
 	get_task_struct(task);
 	llist_add(&task->cg_dead_lnode, this_cpu_ptr(&cgrp_dead_tasks));
 	irq_work_queue(this_cpu_ptr(&cgrp_dead_tasks_iwork));
@@ -7059,6 +7063,7 @@ static void __init cgroup_rt_init(void) {}
 
 void cgroup_task_dead(struct task_struct *task)
 {
+	trace_printk(KERN_INFO "%s: task = %px (%s)\n", __func__, task, task->comm);
 	do_cgroup_task_dead(task);
 }
 #endif	/* CONFIG_PREEMPT_RT */
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 854984967fe2..19b130b831bc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5078,6 +5078,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
 	struct rq *rq = this_rq();
 	struct mm_struct *mm = rq->prev_mm;
 	unsigned int prev_state;
+	trace_printk(KERN_INFO "%s 0: %px (%s)\n", __func__, prev, prev->comm);
 
 	/*
 	 * The previous task will have left us with a preempt_count of 2
@@ -5153,6 +5154,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
 		 * visible to SCX schedulers.
 		 */
 		sched_ext_dead(prev);
+		trace_printk(KERN_INFO "%s 1: %px (%s)\n", __func__, prev, prev->comm);
 		cgroup_task_dead(prev);
 
 		/* Task is done with its stack. */
@@ -5202,6 +5204,7 @@ static __always_inline struct rq *
 context_switch(struct rq *rq, struct task_struct *prev,
 	       struct task_struct *next, struct rq_flags *rf)
 {
+	trace_printk(KERN_INFO "%s 0: %px (%s)\n", __func__, prev, prev->comm);
 	prepare_task_switch(rq, prev, next);
 
 	/*


This if from PREEMPT_RT log, there*s a long pause in which cgroup_task_dead() is not called
59366:           <...>-3209    [001] d..2.    33.110392: 0xffffffffa36c309b: 6context_switch 0: ffff933eb264a180 (reboot)
[...]
112455:          <idle>-0       [006] ...1.    40.503766: 0xffffffffa2da570c: 6cgroup_task_dead: task = ffff933f1885c300 ((udev-worker))
[...] no call to cgroup_task_dead() here, just finish_task_switch() and context_switch()
217571:          <idle>-0       [010] ...1.   125.282118: 0xffffffffa2da570c: 6cgroup_task_dead: task = ffff933e94fae480 (systemd)
[...]
274103          <idle>-0       [014] d..2.   130.157472: 0xffffffffa2cef125: 6finish_task_switch 0: ffff933e815e10c0 (ksoftirqd/14)


This is other log (no pause here, just the first messages after reboot is initiated and
the last message, to show duration of shutdown):

58029:           <...>-2975    [003] d..2.    33.564291: 0xffffffff934b3e89: 6context_switch 0: ffff88e1e0302180 (reboot)
[...]
107700           <...>-1       [000] d..2.    37.352191: 0xffffffff92aee9c5: 6finish_task_switch 0: ffffffff93c12980 (swapper/0)

The complete logs are here:
https://gitlab.freedesktop.org/spasswolf/pastebin/-/issues/2

Bert Karwatzki