Re: WARN_ON_ONCE() in process_one_work()?

[Tejun Heo]

Hello, Paul.

Sorry about the late reply.

On Wed, Jun 20, 2018 at 12:29:01PM -0700, Paul E. McKenney wrote:
> I have hit this WARN_ON_ONCE() in process_one_work:
> 
> 	WARN_ON_ONCE(!(pool->flags & POOL_DISASSOCIATED) &&
> 		     raw_smp_processor_id() != pool->cpu);
> 
> This looks like it is my rcu_gp workqueue (see splat below), and it
> appears to be intermittent.  This happens on rcutorture scenario SRCU-N,
> which does random CPU-hotplug operations (in case that helps).
> 
> Is this related to the recent addition of WQ_MEM_RECLAIM?  Either way,
> what should I do to further debug this?

Hmm... I checked the code paths but couldn't spot anything suspicious.
Can you please apply the following patch and see whether it triggers
before hitting the warn and if so report what it says?

Thanks.

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 0db8938fbb23..81caab9643b2 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -79,6 +79,15 @@ static struct lockdep_map cpuhp_state_up_map =
 static struct lockdep_map cpuhp_state_down_map =
 	STATIC_LOCKDEP_MAP_INIT("cpuhp_state-down", &cpuhp_state_down_map);
 
+int cpuhp_current_state(int cpu)
+{
+	return per_cpu_ptr(&cpuhp_state, cpu)->state;
+}
+
+int cpuhp_target_state(int cpu)
+{
+	return per_cpu_ptr(&cpuhp_state, cpu)->target;
+}
 
 static inline void cpuhp_lock_acquire(bool bringup)
 {
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 78b192071ef7..365cf6342808 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1712,6 +1712,9 @@ static struct worker *alloc_worker(int node)
 	return worker;
 }
 
+int cpuhp_current_state(int cpu);
+int cpuhp_target_state(int cpu);
+
 /**
  * worker_attach_to_pool() - attach a worker to a pool
  * @worker: worker to be attached
@@ -1724,13 +1727,20 @@ static struct worker *alloc_worker(int node)
 static void worker_attach_to_pool(struct worker *worker,
 				   struct worker_pool *pool)
 {
+	int ret;
+
 	mutex_lock(&wq_pool_attach_mutex);
 
 	/*
 	 * set_cpus_allowed_ptr() will fail if the cpumask doesn't have any
 	 * online CPUs.  It'll be re-applied when any of the CPUs come up.
 	 */
-	set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
+	ret = set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
+	if (ret && pool->cpu >= 0 && worker->rescue_wq)
+		printk("XXX rescuer failed to attach: ret=%d pool=%d this_cpu=%d target_cpu=%d cpuhp_state=%d chuhp_target=%d\n",
+		       ret, pool->id, raw_smp_processor_id(), pool->cpu,
+		       cpuhp_current_state(pool->cpu),
+		       cpuhp_target_state(pool->cpu));
 
 	/*
 	 * The wq_pool_attach_mutex ensures %POOL_DISASSOCIATED remains