workqueue: Make per-cpu pool_workqueues allocated and released like unbound ones

Currently, all per-cpu pwq's (pool_workqueue's) are allocated directly
through a per-cpu allocation and thus, unlike unbound workqueues, not
reference counted. This difference in lifetime management between the two
types is a bit confusing.

Unbound workqueues are currently accessed through wq->numa_pwq_tbl[] which
isn't suitiable for the planned CPU locality related improvements. The plan
is to unify pwq handling across per-cpu and unbound workqueues so that
they're always accessed through wq->cpu_pwq.

In preparation, this patch makes per-cpu pwq's to be allocated, reference
counted and released the same way as unbound pwq's. wq->cpu_pwq now holds
pointers to pwq's instead of containing them directly.

pwq_unbound_release_workfn() is renamed to pwq_release_workfn() as it's now
also used for per-cpu work items.
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -258,11 +258,11 @@ struct pool_workqueue {
 
 	/*
 	 * Release of unbound pwq is punted to a kthread_worker. See put_pwq()
-	 * and pwq_unbound_release_workfn() for details. pool_workqueue itself
-	 * is also RCU protected so that the first pwq can be determined without
+	 * and pwq_release_workfn() for details. pool_workqueue itself is also
+	 * RCU protected so that the first pwq can be determined without
 	 * grabbing wq->mutex.
 	 */
-	struct kthread_work	unbound_release_work;
+	struct kthread_work	release_work;
 	struct rcu_head		rcu;
 } __aligned(1 << WORK_STRUCT_FLAG_BITS);
 
@@ -321,7 +321,7 @@ struct workqueue_struct {
 
 	/* hot fields used during command issue, aligned to cacheline */
 	unsigned int		flags ____cacheline_aligned; /* WQ: WQ_* flags */
-	struct pool_workqueue __percpu *cpu_pwq; /* I: per-cpu pwqs */
+	struct pool_workqueue __percpu **cpu_pwq; /* I: per-cpu pwqs */
 	struct pool_workqueue __rcu *numa_pwq_tbl[]; /* PWR: unbound pwqs indexed by node */
 };
 
@@ -1370,13 +1370,11 @@ static void put_pwq(struct pool_workqueue *pwq)
 	lockdep_assert_held(&pwq->pool->lock);
 	if (likely(--pwq->refcnt))
 		return;
-	if (WARN_ON_ONCE(!(pwq->wq->flags & WQ_UNBOUND)))
-		return;
 	/*
 	 * @pwq can't be released under pool->lock, bounce to a dedicated
 	 * kthread_worker to avoid A-A deadlocks.
 	 */
-	kthread_queue_work(pwq_release_worker, &pwq->unbound_release_work);
+	kthread_queue_work(pwq_release_worker, &pwq->release_work);
 }
 
 /**
@@ -1685,7 +1683,7 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
 	} else {
 		if (req_cpu == WORK_CPU_UNBOUND)
 			cpu = raw_smp_processor_id();
-		pwq = per_cpu_ptr(wq->cpu_pwq, cpu);
+		pwq = *per_cpu_ptr(wq->cpu_pwq, cpu);
 	}
 
 	pool = pwq->pool;
@@ -4004,31 +4002,30 @@ static void rcu_free_pwq(struct rcu_head *rcu)
  * Scheduled on pwq_release_worker by put_pwq() when an unbound pwq hits zero
  * refcnt and needs to be destroyed.
  */
-static void pwq_unbound_release_workfn(struct kthread_work *work)
+static void pwq_release_workfn(struct kthread_work *work)
 {
 	struct pool_workqueue *pwq = container_of(work, struct pool_workqueue,
-						  unbound_release_work);
+						  release_work);
 	struct workqueue_struct *wq = pwq->wq;
 	struct worker_pool *pool = pwq->pool;
 	bool is_last = false;
 
 	/*
-	 * when @pwq is not linked, it doesn't hold any reference to the
+	 * When @pwq is not linked, it doesn't hold any reference to the
 	 * @wq, and @wq is invalid to access.
 	 */
 	if (!list_empty(&pwq->pwqs_node)) {
-		if (WARN_ON_ONCE(!(wq->flags & WQ_UNBOUND)))
-			return;
-
 		mutex_lock(&wq->mutex);
 		list_del_rcu(&pwq->pwqs_node);
 		is_last = list_empty(&wq->pwqs);
 		mutex_unlock(&wq->mutex);
 	}
 
-	mutex_lock(&wq_pool_mutex);
-	put_unbound_pool(pool);
-	mutex_unlock(&wq_pool_mutex);
+	if (wq->flags & WQ_UNBOUND) {
+		mutex_lock(&wq_pool_mutex);
+		put_unbound_pool(pool);
+		mutex_unlock(&wq_pool_mutex);
+	}
 
 	call_rcu(&pwq->rcu, rcu_free_pwq);
 
@@ -4112,8 +4109,7 @@ static void init_pwq(struct pool_workqueue *pwq, struct workqueue_struct *wq,
 	INIT_LIST_HEAD(&pwq->inactive_works);
 	INIT_LIST_HEAD(&pwq->pwqs_node);
 	INIT_LIST_HEAD(&pwq->mayday_node);
-	kthread_init_work(&pwq->unbound_release_work,
-			  pwq_unbound_release_workfn);
+	kthread_init_work(&pwq->release_work, pwq_release_workfn);
 }
 
 /* sync @pwq with the current state of its associated wq and link it */
@@ -4514,20 +4510,25 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq)
 	int cpu, ret;
 
 	if (!(wq->flags & WQ_UNBOUND)) {
-		wq->cpu_pwq = alloc_percpu(struct pool_workqueue);
+		wq->cpu_pwq = alloc_percpu(struct pool_workqueue *);
 		if (!wq->cpu_pwq)
-			return -ENOMEM;
+			goto enomem;
 
 		for_each_possible_cpu(cpu) {
-			struct pool_workqueue *pwq =
+			struct pool_workqueue **pwq_p =
 				per_cpu_ptr(wq->cpu_pwq, cpu);
-			struct worker_pool *cpu_pools =
-				per_cpu(cpu_worker_pools, cpu);
+			struct worker_pool *pool =
+				&(per_cpu_ptr(cpu_worker_pools, cpu)[highpri]);
 
-			init_pwq(pwq, wq, &cpu_pools[highpri]);
+			*pwq_p = kmem_cache_alloc_node(pwq_cache, GFP_KERNEL,
+						       pool->node);
+			if (!*pwq_p)
+				goto enomem;
+
+			init_pwq(*pwq_p, wq, pool);
 
 			mutex_lock(&wq->mutex);
-			link_pwq(pwq);
+			link_pwq(*pwq_p);
 			mutex_unlock(&wq->mutex);
 		}
 		return 0;
@@ -4546,6 +4547,15 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq)
 	cpus_read_unlock();
 
 	return ret;
+
+enomem:
+	if (wq->cpu_pwq) {
+		for_each_possible_cpu(cpu)
+			kfree(*per_cpu_ptr(wq->cpu_pwq, cpu));
+		free_percpu(wq->cpu_pwq);
+		wq->cpu_pwq = NULL;
+	}
+	return -ENOMEM;
 }
 
 static int wq_clamp_max_active(int max_active, unsigned int flags,
@@ -4719,7 +4729,7 @@ static bool pwq_busy(struct pool_workqueue *pwq)
 void destroy_workqueue(struct workqueue_struct *wq)
 {
 	struct pool_workqueue *pwq;
-	int node;
+	int cpu, node;
 
 	/*
 	 * Remove it from sysfs first so that sanity check failure doesn't
@@ -4779,12 +4789,8 @@ void destroy_workqueue(struct workqueue_struct *wq)
 	mutex_unlock(&wq_pool_mutex);
 
 	if (!(wq->flags & WQ_UNBOUND)) {
-		wq_unregister_lockdep(wq);
-		/*
-		 * The base ref is never dropped on per-cpu pwqs.  Directly
-		 * schedule RCU free.
-		 */
-		call_rcu(&wq->rcu, rcu_free_wq);
+		for_each_possible_cpu(cpu)
+			put_pwq_unlocked(*per_cpu_ptr(wq->cpu_pwq, cpu));
 	} else {
 		/*
 		 * We're the sole accessor of @wq at this point.  Directly
@@ -4901,7 +4907,7 @@ bool workqueue_congested(int cpu, struct workqueue_struct *wq)
 		cpu = smp_processor_id();
 
 	if (!(wq->flags & WQ_UNBOUND))
-		pwq = per_cpu_ptr(wq->cpu_pwq, cpu);
+		pwq = *per_cpu_ptr(wq->cpu_pwq, cpu);
 	else
 		pwq = unbound_pwq_by_node(wq, cpu_to_node(cpu));