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 <[email protected]>
---
kernel/workqueue.c | 70 ++++++++++++++++++++++++++++++++++++++--------
1 file changed, 58 insertions(+), 12 deletions(-)
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index c8b1466a9c070..da6396f513102 100644
--- 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 */
@@ -1904,7 +1905,7 @@ 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);
@@ -1993,21 +1994,44 @@ 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) ||
@@ -2018,9 +2042,10 @@ 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);
}
/*
@@ -2062,11 +2087,24 @@ static void idle_worker_timeout(struct timer_list *t)
/*
* idle_cull_fn - cull workers that have been idle for too long.
+ *
+ * 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)) {
@@ -2081,10 +2119,12 @@ 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)
@@ -2448,12 +2488,12 @@ static int worker_thread(void *__worker)
/* 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;
}
@@ -3527,6 +3567,7 @@ 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);
@@ -3627,8 +3668,11 @@ static bool wq_manager_inactive(struct worker_pool *pool)
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)
@@ -3651,17 +3695,19 @@ static void put_unbound_pool(struct worker_pool *pool)
* Because of how wq_manager_inactive() works, we will hold the
* spinlock after a successful wait.
*/
+ mutex_lock(&wq_pool_attach_mutex);
rcuwait_wait_event(&manager_wait, wq_manager_inactive(pool),
TASK_UNINTERRUPTIBLE);
pool->flags |= POOL_MANAGER_ACTIVE;
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);
--
2.31.1
On Tue, Nov 29, 2022 at 2:31 AM Valentin Schneider <[email protected]> wrote:
> @@ -3627,8 +3668,11 @@ static bool wq_manager_inactive(struct worker_pool *pool)
> 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)
> @@ -3651,17 +3695,19 @@ static void put_unbound_pool(struct worker_pool *pool)
> * Because of how wq_manager_inactive() works, we will hold the
> * spinlock after a successful wait.
> */
> + mutex_lock(&wq_pool_attach_mutex);
> rcuwait_wait_event(&manager_wait, wq_manager_inactive(pool),
> TASK_UNINTERRUPTIBLE);
> pool->flags |= POOL_MANAGER_ACTIVE;
Hello, Valentin
I'm afraid it might deadlock here.
If put_unbound_pool() is called while manage_workers() is sleeping
on allocating memory, put_unbound_pool() will get the wq_pool_attach_mutex
earlier than the manager which prevents the manager from getting the
lock to attach the newly created worker and deadlock.
I think mutex_lock(&wq_pool_attach_mutex) can be moved into
wq_manager_inactive(), and handle it in the same way as pool->lock.
>
> 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);
>
> --
> 2.31.1
>
On 01/12/22 11:01, Lai Jiangshan wrote:
> On Tue, Nov 29, 2022 at 2:31 AM Valentin Schneider <[email protected]> wrote:
>
>> @@ -3627,8 +3668,11 @@ static bool wq_manager_inactive(struct worker_pool *pool)
>> 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)
>> @@ -3651,17 +3695,19 @@ static void put_unbound_pool(struct worker_pool *pool)
>> * Because of how wq_manager_inactive() works, we will hold the
>> * spinlock after a successful wait.
>> */
>> + mutex_lock(&wq_pool_attach_mutex);
>> rcuwait_wait_event(&manager_wait, wq_manager_inactive(pool),
>> TASK_UNINTERRUPTIBLE);
>> pool->flags |= POOL_MANAGER_ACTIVE;
>
> Hello, Valentin
>
> I'm afraid it might deadlock here.
>
> If put_unbound_pool() is called while manage_workers() is sleeping
> on allocating memory, put_unbound_pool() will get the wq_pool_attach_mutex
> earlier than the manager which prevents the manager from getting the
> lock to attach the newly created worker and deadlock.
>
Well spotted, I can see it now.
> I think mutex_lock(&wq_pool_attach_mutex) can be moved into
> wq_manager_inactive(), and handle it in the same way as pool->lock.
>
That looks sane enough, I'll try to tweak my tests to get the manager
involved to test this out. Thanks!