Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jan 14, 2021 at 01:21:26PM +0000, Valentin Schneider wrote:
> On 14/01/21 14:12, Peter Zijlstra wrote:

> > -		WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
> > -						  pool->attrs->cpumask) < 0);
> > +		kthread_park(worker->task);
> 
> Don't we still need an affinity change here, to undo what was done in
> unbind_workers()?
> 
> Would something like
> 
>   __kthread_bind_mask(worker->task, pool->attrs->cpumask, TASK_PARKED)
> 
> even work?
> 
> >               kthread_set_per_cpu(worker->task, true);
> > +		kthread_unpark(worker->task);

Nope, look at what kthread_unpark() does, what was missing was assigning
kthread->cpu though.

The below seems to actually work. Rescuer is still a problem though.

---
 include/linux/kthread.h |  2 +-
 kernel/kthread.c        | 14 ++++++++------
 kernel/sched/core.c     | 19 ++++++++++++++++++-
 kernel/smpboot.c        |  2 +-
 kernel/workqueue.c      | 22 +++++++++++++---------
 5 files changed, 41 insertions(+), 18 deletions(-)

diff --git a/include/linux/kthread.h b/include/linux/kthread.h
index fdd5a52e35d8..2484ed97e72f 100644
--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -33,7 +33,7 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
 					  unsigned int cpu,
 					  const char *namefmt);
 
-void kthread_set_per_cpu(struct task_struct *k, bool set);
+void kthread_set_per_cpu(struct task_struct *k, int cpu);
 bool kthread_is_per_cpu(struct task_struct *k);
 
 /**
diff --git a/kernel/kthread.c b/kernel/kthread.c
index bead90275d2b..e0e4a423f184 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -497,19 +497,21 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
 	return p;
 }
 
-void kthread_set_per_cpu(struct task_struct *k, bool set)
+void kthread_set_per_cpu(struct task_struct *k, int cpu)
 {
 	struct kthread *kthread = to_kthread(k);
 	if (!kthread)
 		return;
 
-	if (set) {
-		WARN_ON_ONCE(!(k->flags & PF_NO_SETAFFINITY));
-		WARN_ON_ONCE(k->nr_cpus_allowed != 1);
-		set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
-	} else {
+	WARN_ON_ONCE(!(k->flags & PF_NO_SETAFFINITY));
+
+	if (cpu < 0) {
 		clear_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
+		return;
 	}
+
+	kthread->cpu = cpu;
+	set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
 }
 
 bool kthread_is_per_cpu(struct task_struct *k)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 60b257d845fa..c2fdeeb6af2b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7589,7 +7589,24 @@ int sched_cpu_dying(unsigned int cpu)
 	sched_tick_stop(cpu);
 
 	rq_lock_irqsave(rq, &rf);
-	BUG_ON(rq->nr_running != 1 || rq_has_pinned_tasks(rq));
+	if (rq->nr_running != 1 || rq_has_pinned_tasks(rq)) {
+		struct task_struct *g, *p;
+
+		pr_crit("CPU%d nr_running=%d\n", cpu, rq->nr_running);
+		rcu_read_lock();
+		for_each_process_thread(g, p) {
+			if (task_cpu(p) != cpu)
+				continue;
+
+			if (!task_on_rq_queued(p))
+				continue;
+
+			pr_crit("\tp=%s\n", p->comm);
+		}
+		rcu_read_unlock();
+
+		WARN_ON_ONCE(1);
+	}
 	rq_unlock_irqrestore(rq, &rf);
 
 	calc_load_migrate(rq);
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index b0abe575a524..f25208e8df83 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -188,7 +188,7 @@ __smpboot_create_thread(struct smp_hotplug_thread *ht, unsigned int cpu)
 		kfree(td);
 		return PTR_ERR(tsk);
 	}
-	kthread_set_per_cpu(tsk, true);
+	kthread_set_per_cpu(tsk, cpu);
 	/*
 	 * Park the thread so that it could start right on the CPU
 	 * when it is available.
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index ec0771e4a3fb..b518fd67a792 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1862,7 +1862,7 @@ static void worker_attach_to_pool(struct worker *worker,
 	if (pool->flags & POOL_DISASSOCIATED)
 		worker->flags |= WORKER_UNBOUND;
 	else
-		kthread_set_per_cpu(worker->task, true);
+		kthread_set_per_cpu(worker->task, pool->cpu);
 
 	list_add_tail(&worker->node, &pool->workers);
 	worker->pool = pool;
@@ -1885,7 +1885,7 @@ static void worker_detach_from_pool(struct worker *worker)
 
 	mutex_lock(&wq_pool_attach_mutex);
 
-	kthread_set_per_cpu(worker->task, false);
+	kthread_set_per_cpu(worker->task, -1);
 	list_del(&worker->node);
 	worker->pool = NULL;
 
@@ -2371,6 +2371,7 @@ static int worker_thread(void *__worker)
 	/* tell the scheduler that this is a workqueue worker */
 	set_pf_worker(true);
 woke_up:
+	kthread_parkme();
 	raw_spin_lock_irq(&pool->lock);
 
 	/* am I supposed to die? */
@@ -2428,7 +2429,7 @@ static int worker_thread(void *__worker)
 			move_linked_works(work, &worker->scheduled, NULL);
 			process_scheduled_works(worker);
 		}
-	} while (keep_working(pool));
+	} while (keep_working(pool) && !kthread_should_park());
 
 	worker_set_flags(worker, WORKER_PREP);
 sleep:
@@ -2440,9 +2441,12 @@ static int worker_thread(void *__worker)
 	 * event.
 	 */
 	worker_enter_idle(worker);
-	__set_current_state(TASK_IDLE);
+	set_current_state(TASK_IDLE);
 	raw_spin_unlock_irq(&pool->lock);
-	schedule();
+
+	if (!kthread_should_park())
+		schedule();
+
 	goto woke_up;
 }
 
@@ -4923,7 +4927,7 @@ static void unbind_workers(int cpu)
 		raw_spin_unlock_irq(&pool->lock);
 
 		for_each_pool_worker(worker, pool) {
-			kthread_set_per_cpu(worker->task, false);
+			kthread_set_per_cpu(worker->task, -1);
 			WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, cpu_possible_mask) < 0);
 		}
 
@@ -4978,9 +4982,9 @@ static void rebind_workers(struct worker_pool *pool)
 	 * from CPU_ONLINE, the following shouldn't fail.
 	 */
 	for_each_pool_worker(worker, pool) {
-		WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
-						  pool->attrs->cpumask) < 0);
-		kthread_set_per_cpu(worker->task, true);
+		WARN_ON_ONCE(kthread_park(worker->task) < 0);
+		kthread_set_per_cpu(worker->task, pool->cpu);
+		kthread_unpark(worker->task);
 	}
 
 	raw_spin_lock_irq(&pool->lock);