Subject: Re: powerpc 5.10-rcN boot failures with RCU_SCALE_TEST=m

From: Uladzislau Rezki <[email-masked]>
Date: Fri, 4 Dec 2020 21:23:29 +0100

On Thu, Dec 03, 2020 at 03:34:45PM +0100, Uladzislau Rezki wrote:
> On Thu, Dec 03, 2020 at 05:22:20PM +1100, Michael Ellerman wrote:
> > Uladzislau Rezki <[email-masked]> writes:
> > > On Thu, Dec 03, 2020 at 01:03:32AM +1100, Michael Ellerman wrote:
> > ...
> > >> 
> > >> The SMP bringup stalls because _cpu_up() is blocked trying to take
> > >> cpu_hotplug_lock for writing:
> > >> 
> > >> [  401.403132][    T0] task:swapper/0       state:D stack:12512 pid:    1 ppid:     0 flags:0x00000800
> > >> [  401.403502][    T0] Call Trace:
> > >> [  401.403907][    T0] [c0000000062c37d0] [c0000000062c3830] 0xc0000000062c3830 (unreliable)
> > >> [  401.404068][    T0] [c0000000062c39b0] [c000000000019d70] __switch_to+0x2e0/0x4a0
> > >> [  401.404189][    T0] [c0000000062c3a10] [c000000000b87228] __schedule+0x288/0x9b0
> > >> [  401.404257][    T0] [c0000000062c3ad0] [c000000000b879b8] schedule+0x68/0x120
> > >> [  401.404324][    T0] [c0000000062c3b00] [c000000000184ad4] percpu_down_write+0x164/0x170
> > >> [  401.404390][    T0] [c0000000062c3b50] [c000000000116b68] _cpu_up+0x68/0x280
> > >> [  401.404475][    T0] [c0000000062c3bb0] [c000000000116e70] cpu_up+0xf0/0x140
> > >> [  401.404546][    T0] [c0000000062c3c30] [c00000000011776c] bringup_nonboot_cpus+0xac/0xf0
> > >> [  401.404643][    T0] [c0000000062c3c80] [c000000000eea1b8] smp_init+0x40/0xcc
> > >> [  401.404727][    T0] [c0000000062c3ce0] [c000000000ec43dc] kernel_init_freeable+0x1e0/0x3a0
> > >> [  401.404799][    T0] [c0000000062c3db0] [c000000000011ec4] kernel_init+0x24/0x150
> > >> [  401.404958][    T0] [c0000000062c3e20] [c00000000000daf0] ret_from_kernel_thread+0x5c/0x6c
> > >> 
> > >> It can't get it because kprobe_optimizer() has taken it for read and is now
> > >> blocked waiting for synchronize_rcu_tasks():
> > >> 
> > >> [  401.418808][    T0] task:kworker/0:1     state:D stack:13392 pid:   12 ppid:     2 flags:0x00000800
> > >> [  401.418951][    T0] Workqueue: events kprobe_optimizer
> > >> [  401.419078][    T0] Call Trace:
> > >> [  401.419121][    T0] [c0000000062ef650] [c0000000062ef710] 0xc0000000062ef710 (unreliable)
> > >> [  401.419213][    T0] [c0000000062ef830] [c000000000019d70] __switch_to+0x2e0/0x4a0
> > >> [  401.419281][    T0] [c0000000062ef890] [c000000000b87228] __schedule+0x288/0x9b0
> > >> [  401.419347][    T0] [c0000000062ef950] [c000000000b879b8] schedule+0x68/0x120
> > >> [  401.419415][    T0] [c0000000062ef980] [c000000000b8e664] schedule_timeout+0x2a4/0x340
> > >> [  401.419484][    T0] [c0000000062efa80] [c000000000b894ec] wait_for_completion+0x9c/0x170
> > >> [  401.419552][    T0] [c0000000062efae0] [c0000000001ac85c] __wait_rcu_gp+0x19c/0x210
> > >> [  401.419619][    T0] [c0000000062efb40] [c0000000001ac90c] synchronize_rcu_tasks_generic+0x3c/0x70
> > >> [  401.419690][    T0] [c0000000062efbe0] [c00000000022a3dc] kprobe_optimizer+0x1dc/0x470
> > >> [  401.419757][    T0] [c0000000062efc60] [c000000000136684] process_one_work+0x2f4/0x530
> > >> [  401.419823][    T0] [c0000000062efd20] [c000000000138d28] worker_thread+0x78/0x570
> > >> [  401.419891][    T0] [c0000000062efdb0] [c000000000142424] kthread+0x194/0x1a0
> > >> [  401.419976][    T0] [c0000000062efe20] [c00000000000daf0] ret_from_kernel_thread+0x5c/0x6c
> > >> 
> > >> But why is the synchronize_rcu_tasks() not completing?
OK. Seems i have understood why the synchronize_rcu_tasks() is not doing progress
waiting on complition. Actually the GP kthreads are not spawned by the time when
early_initcall callbacks gets invoked.

It means that callbacks will not be processed because GP kthreads do not exist,
so wakeme_after_rcu() is not invoked, thus does not signal about that a grace
period has elapsed.

<snip>
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 35bdcfd84d42..c5422bba7fe7 100644  
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -241,7 +241,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
        }
 }

-/* Spawn RCU-tasks grace-period kthread, e.g., at core_initcall() time. */
+/* Spawn RCU-tasks grace-period kthread at early_initcall() time. */
 static void __init rcu_spawn_tasks_kthread_generic(struct rcu_tasks *rtp)
 {
        struct task_struct *t;
@@ -564,7 +564,7 @@ static int __init rcu_spawn_tasks_kthread(void)
        rcu_spawn_tasks_kthread_generic(&rcu_tasks);
        return 0;
 }
-core_initcall(rcu_spawn_tasks_kthread);
+early_initcall(rcu_spawn_tasks_kthread);
 
 #if !defined(CONFIG_TINY_RCU)
 void show_rcu_tasks_classic_gp_kthread(void)
@@ -692,7 +692,7 @@ static int __init rcu_spawn_tasks_rude_kthread(void)
        rcu_spawn_tasks_kthread_generic(&rcu_tasks_rude);
        return 0;
 }
-core_initcall(rcu_spawn_tasks_rude_kthread);
+early_initcall(rcu_spawn_tasks_rude_kthread);
 
 #if !defined(CONFIG_TINY_RCU)
 void show_rcu_tasks_rude_gp_kthread(void)
@@ -1193,7 +1193,7 @@ static int __init rcu_spawn_tasks_trace_kthread(void)
        rcu_spawn_tasks_kthread_generic(&rcu_tasks_trace);
        return 0;
 }
-core_initcall(rcu_spawn_tasks_trace_kthread);
+early_initcall(rcu_spawn_tasks_trace_kthread);
 
 #if !defined(CONFIG_TINY_RCU)
 void show_rcu_tasks_trace_gp_kthread(void)
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 39334d2d2b37..a251fc705abd 100644  
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -133,7 +133,7 @@ EXPORT_SYMBOL(rcu_read_lock_sched_held);
  * that if the user specifies both rcu_expedited and rcu_normal, then
  * rcu_normal wins.  (Except during the time period during boot from
  * when the first task is spawned until the rcu_set_runtime_mode()
- * core_initcall() is invoked, at which point everything is expedited.)
+ * early_initcall() is invoked, at which point everything is expedited.)
  */

ol rcu_gp_is_normal(void)
 {
@@ -235,7 +235,7 @@ static int __init rcu_set_runtime_mode(void)
        rcu_test_sync_prims();
        return 0;
 }
-core_initcall(rcu_set_runtime_mode);
+early_initcall(rcu_set_runtime_mode);   

 #endif /* #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU) */
<snip>

Appreciate if you can test on your setup above patch!

--
Vlad Rezki