workqueue: Unbind kworkers before sending them to exit()

It has been reported that isolated CPUs can suffer from interference due to
per-CPU kworkers waking up just to die.

A surge of workqueue activity during initial setup of a latency-sensitive
application (refresh_vm_stats() being one of the culprits) can cause extra
per-CPU kworkers to be spawned. Then, said latency-sensitive task can be
running merrily on an isolated CPU only to be interrupted sometime later by
a kworker marked for death (cf. IDLE_WORKER_TIMEOUT, 5 minutes after last
kworker activity).

Prevent this by affining kworkers to the wq_unbound_cpumask (which doesn't
contain isolated CPUs, cf. HK_TYPE_WQ) before waking them up after marking
them with WORKER_DIE.

Changing the affinity does require a sleepable context, leverage the newly
introduced pool->idle_cull_work to get that.

Remove dying workers from pool->workers and keep track of them in a
separate list. This intentionally prevents for_each_loop_worker() from
iterating over workers that are marked for death.

Rename destroy_worker() to set_working_dying() to better reflect its
effects and relationship with wake_dying_workers().

Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index a826956..5dc67aa 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -179,6 +179,7 @@ struct worker_pool {
 
        struct worker           *manager;       /* L: purely informational */
        struct list_head        workers;        /* A: attached workers */
+       struct list_head        dying_workers;  /* A: workers about to die */
        struct completion       *detach_completion; /* all workers detached */
 
        struct ida              worker_ida;     /* worker IDs for task name */
@@ -1906,7 +1907,7 @@ static void worker_detach_from_pool(struct worker *worker)
        list_del(&worker->node);
        worker->pool = NULL;
 
-       if (list_empty(&pool->workers))
+       if (list_empty(&pool->workers) && list_empty(&pool->dying_workers))
                detach_completion = pool->detach_completion;
        mutex_unlock(&wq_pool_attach_mutex);
 
@@ -1995,21 +1996,44 @@ static void rebind_worker(struct worker *worker, struct worker_pool *pool)
        WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask) < 0);
 }
 
+static void wake_dying_workers(struct list_head *cull_list)
+{
+       struct worker *worker, *tmp;
+
+       list_for_each_entry_safe(worker, tmp, cull_list, entry) {
+               list_del_init(&worker->entry);
+               unbind_worker(worker);
+               /*
+                * If the worker was somehow already running, then it had to be
+                * in pool->idle_list when set_worker_dying() happened or we
+                * wouldn't have gotten here.
+                *
+                * Thus, the worker must either have observed the WORKER_DIE
+                * flag, or have set its state to TASK_IDLE. Either way, the
+                * below will be observed by the worker and is safe to do
+                * outside of pool->lock.
+                */
+               wake_up_process(worker->task);
+       }
+}
+
 /**
- * destroy_worker - destroy a workqueue worker
+ * set_worker_dying - Tag a worker for destruction
  * @worker: worker to be destroyed
+ * @list: transfer worker away from its pool->idle_list and into list
  *
- * Destroy @worker and adjust @pool stats accordingly.  The worker should
- * be idle.
+ * Tag @worker for destruction and adjust @pool stats accordingly.  The worker
+ * should be idle.
  *
  * CONTEXT:
  * raw_spin_lock_irq(pool->lock).
  */
-static void destroy_worker(struct worker *worker)
+static void set_worker_dying(struct worker *worker, struct list_head *list)
 {
        struct worker_pool *pool = worker->pool;
 
        lockdep_assert_held(&pool->lock);
+       lockdep_assert_held(&wq_pool_attach_mutex);
 
        /* sanity check frenzy */
        if (WARN_ON(worker->current_work) ||
@@ -2020,9 +2044,10 @@ static void destroy_worker(struct worker *worker)
        pool->nr_workers--;
        pool->nr_idle--;
 
-       list_del_init(&worker->entry);
        worker->flags |= WORKER_DIE;
-       wake_up_process(worker->task);
+
+       list_move(&worker->entry, list);
+       list_move(&worker->node, &pool->dying_workers);
 }
 
 /**
@@ -2069,11 +2094,24 @@ static void idle_worker_timeout(struct timer_list *t)
  *
  * This goes through a pool's idle workers and gets rid of those that have been
  * idle for at least IDLE_WORKER_TIMEOUT seconds.
+ *
+ * We don't want to disturb isolated CPUs because of a pcpu kworker being
+ * culled, so this also resets worker affinity. This requires a sleepable
+ * context, hence the split between timer callback and work item.
  */
 static void idle_cull_fn(struct work_struct *work)
 {
        struct worker_pool *pool = container_of(work, struct worker_pool, idle_cull_work);
+       struct list_head cull_list;
 
+       INIT_LIST_HEAD(&cull_list);
+       /*
+        * Grabbing wq_pool_attach_mutex here ensures an already-running worker
+        * cannot proceed beyong worker_detach_from_pool() in its self-destruct
+        * path. This is required as a previously-preempted worker could run after
+        * set_worker_dying() has happened but before wake_dying_workers() did.
+        */
+       mutex_lock(&wq_pool_attach_mutex);
        raw_spin_lock_irq(&pool->lock);
 
        while (too_many_workers(pool)) {
@@ -2088,10 +2126,12 @@ static void idle_cull_fn(struct work_struct *work)
                        break;
                }
 
-               destroy_worker(worker);
+               set_worker_dying(worker, &cull_list);
        }
 
        raw_spin_unlock_irq(&pool->lock);
+       wake_dying_workers(&cull_list);
+       mutex_unlock(&wq_pool_attach_mutex);
 }
 
 static void send_mayday(struct work_struct *work)
@@ -2455,12 +2495,12 @@ woke_up:
        /* am I supposed to die? */
        if (unlikely(worker->flags & WORKER_DIE)) {
                raw_spin_unlock_irq(&pool->lock);
-               WARN_ON_ONCE(!list_empty(&worker->entry));
                set_pf_worker(false);
 
                set_task_comm(worker->task, "kworker/dying");
                ida_free(&pool->worker_ida, worker->id);
                worker_detach_from_pool(worker);
+               WARN_ON_ONCE(!list_empty(&worker->entry));
                kfree(worker);
                return 0;
        }
@@ -3534,6 +3574,7 @@ static int init_worker_pool(struct worker_pool *pool)
        timer_setup(&pool->mayday_timer, pool_mayday_timeout, 0);
 
        INIT_LIST_HEAD(&pool->workers);
+       INIT_LIST_HEAD(&pool->dying_workers);
 
        ida_init(&pool->worker_ida);
        INIT_HLIST_NODE(&pool->hash_node);
@@ -3622,8 +3663,11 @@ static void rcu_free_pool(struct rcu_head *rcu)
 static void put_unbound_pool(struct worker_pool *pool)
 {
        DECLARE_COMPLETION_ONSTACK(detach_completion);
+       struct list_head cull_list;
        struct worker *worker;
 
+       INIT_LIST_HEAD(&cull_list);
+
        lockdep_assert_held(&wq_pool_mutex);
 
        if (--pool->refcnt)
@@ -3656,21 +3700,25 @@ static void put_unbound_pool(struct worker_pool *pool)
                rcuwait_wait_event(&manager_wait,
                                   !(pool->flags & POOL_MANAGER_ACTIVE),
                                   TASK_UNINTERRUPTIBLE);
+
+               mutex_lock(&wq_pool_attach_mutex);
                raw_spin_lock_irq(&pool->lock);
                if (!(pool->flags & POOL_MANAGER_ACTIVE)) {
                        pool->flags |= POOL_MANAGER_ACTIVE;
                        break;
                }
                raw_spin_unlock_irq(&pool->lock);
+               mutex_unlock(&wq_pool_attach_mutex);
        }
 
        while ((worker = first_idle_worker(pool)))
-               destroy_worker(worker);
+               set_worker_dying(worker, &cull_list);
        WARN_ON(pool->nr_workers || pool->nr_idle);
        raw_spin_unlock_irq(&pool->lock);
 
-       mutex_lock(&wq_pool_attach_mutex);
-       if (!list_empty(&pool->workers))
+       wake_dying_workers(&cull_list);
+
+       if (!list_empty(&pool->workers) || !list_empty(&pool->dying_workers))
                pool->detach_completion = &detach_completion;
        mutex_unlock(&wq_pool_attach_mutex);