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workqueue: Unbind kworkers before sending them to exit()

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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>
This commit is contained in:
Valentin Schneider 2023-01-12 16:14:31 +00:00 committed by Tejun Heo
parent 9ab03be42b
commit e02b931248
1 changed files with 60 additions and 12 deletions
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72
kernel/workqueue.c
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@ -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);