[PATCH 07/10] workqueue: Manually break affinity on hotplug for unbound pool

[Lai Jiangshan <jiangshanlai@gmail.com>]

From: Lai Jiangshan <laijs@linux.alibaba.com>

When all of the CPUs of the unbound pool go down, the scheduler
will break affinity on the workers for us.  We can do it by our own
and don't rely on the scheduler to force break affinity for us.

Fixes: 06249738a41a ("workqueue: Manually break affinity on hotplug")
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 kernel/workqueue.c | 49 ++++++++++++++++++++++++++++++++--------------
 1 file changed, 34 insertions(+), 15 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 878ed83e5908..eea58f77a37b 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -5025,16 +5025,16 @@ static void rebind_workers(struct worker_pool *pool)
 }
 
 /**
- * restore_unbound_workers_cpumask - restore cpumask of unbound workers
+ * update_unbound_workers_cpumask - update cpumask of unbound workers
  * @pool: unbound pool of interest
- * @cpu: the CPU which is coming up
+ * @cpu: the CPU which is coming up or going down
  *
  * An unbound pool may end up with a cpumask which doesn't have any online
- * CPUs.  When a worker of such pool get scheduled, the scheduler resets
- * its cpus_allowed.  If @cpu is in @pool's cpumask which didn't have any
- * online CPU before, cpus_allowed of all its workers should be restored.
+ * CPUs.  We have to reset workers' cpus_allowed of such pool.  And we
+ * restore the workers' cpus_allowed when the pool's cpumask has online
+ * CPU at the first time after reset.
  */
-static void restore_unbound_workers_cpumask(struct worker_pool *pool, int cpu)
+static void update_unbound_workers_cpumask(struct worker_pool *pool, int cpu)
 {
 	static cpumask_t cpumask;
 	struct worker *worker;
@@ -5048,13 +5048,19 @@ static void restore_unbound_workers_cpumask(struct worker_pool *pool, int cpu)
 
 	cpumask_and(&cpumask, pool->attrs->cpumask, wq_online_cpumask);
 
-	/* is @cpu the first one onlined for the @pool? */
-	if (cpumask_weight(&cpumask) > 1)
-		return;
-
-	/* as we're called 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);
+	switch (cpumask_weight(&cpumask)) {
+	case 0: /* @cpu is the last one going down for the @pool. */
+		for_each_pool_worker(worker, pool)
+			WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, cpu_possible_mask) < 0);
+		break;
+	case 1: /* @cpu is the first one onlined for the @pool. */
+		/* as we're called 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);
+		break;
+	default: /* other cases, nothing to do */
+		break;
+	}
 }
 
 int workqueue_prepare_cpu(unsigned int cpu)
@@ -5085,7 +5091,7 @@ int workqueue_online_cpu(unsigned int cpu)
 		if (pool->cpu == cpu)
 			rebind_workers(pool);
 		else if (pool->cpu < 0)
-			restore_unbound_workers_cpumask(pool, cpu);
+			update_unbound_workers_cpumask(pool, cpu);
 
 		mutex_unlock(&wq_pool_attach_mutex);
 	}
@@ -5100,7 +5106,9 @@ int workqueue_online_cpu(unsigned int cpu)
 
 int workqueue_offline_cpu(unsigned int cpu)
 {
+	struct worker_pool *pool;
 	struct workqueue_struct *wq;
+	int pi;
 
 	/* unbinding per-cpu workers should happen on the local CPU */
 	if (WARN_ON(cpu != smp_processor_id()))
@@ -5108,9 +5116,20 @@ int workqueue_offline_cpu(unsigned int cpu)
 
 	unbind_workers(cpu);
 
-	/* update NUMA affinity of unbound workqueues */
 	mutex_lock(&wq_pool_mutex);
 	cpumask_clear_cpu(cpu, wq_online_cpumask);
+
+	/* update CPU affinity of workers of unbound pools */
+	for_each_pool(pool, pi) {
+		mutex_lock(&wq_pool_attach_mutex);
+
+		if (pool->cpu < 0)
+			update_unbound_workers_cpumask(pool, cpu);
+
+		mutex_unlock(&wq_pool_attach_mutex);
+	}
+
+	/* update NUMA affinity of unbound workqueues */
 	list_for_each_entry(wq, &workqueues, list)
 		wq_update_unbound_numa(wq, cpu);
 	mutex_unlock(&wq_pool_mutex);
-- 
2.19.1.6.gb485710b