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Subject[PATCH v7 4/4] workqueue: Unbind kworkers before sending them to exit()
Date
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>
---
kernel/workqueue.c | 79 +++++++++++++++++++++++++++++++++++++++-------
1 file changed, 67 insertions(+), 12 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 11bb657059bcd..45f2187836c67 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 */
@@ -1902,7 +1903,7 @@ 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);

@@ -1991,21 +1992,44 @@ 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) ||
@@ -2016,9 +2040,10 @@ 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);
}

/**
@@ -2065,11 +2090,24 @@ 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)) {
@@ -2084,10 +2122,12 @@ 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)
@@ -2451,12 +2491,12 @@ 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;
}
@@ -3530,6 +3570,7 @@ 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);
@@ -3630,8 +3671,11 @@ 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)
@@ -3658,13 +3702,24 @@ static void put_unbound_pool(struct worker_pool *pool)
TASK_UNINTERRUPTIBLE);
pool->flags |= POOL_MANAGER_ACTIVE;

+ /*
+ * We need to hold wq_pool_attach_mutex() while destroying the workers,
+ * but we can't grab it in rcuwait_wait_event() as it can clobber
+ * current's task state. We can drop pool->lock here as we've set
+ * POOL_MANAGER_ACTIVE, no one else can steal our manager position.
+ */
+ raw_spin_unlock_irq(&pool->lock);
+ mutex_lock(&wq_pool_attach_mutex);
+ raw_spin_lock_irq(&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
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