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Subject[RFC PATCH v3 2/3] workqueue: Unbind workers 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, so get rid of the
pool->idle_timer and use a delayed_work instead. Ensure kworkers do not
free their resources before the new kworker reaper has handled them by
introducing a new struct worker.reaper field - this new field fills in a 4
byte hole in the second cacheline of struct worker.

Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
kernel/workqueue.c | 155 +++++++++++++++++++++++++++++-------
kernel/workqueue_internal.h | 1 +
2 files changed, 126 insertions(+), 30 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 97cc41430a76..28cd58c684ee 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -167,9 +167,9 @@ struct worker_pool {
int nr_workers; /* L: total number of workers */
int nr_idle; /* L: currently idle workers */

- struct list_head idle_list; /* L: list of idle workers */
- struct timer_list idle_timer; /* L: worker idle timeout */
- struct timer_list mayday_timer; /* L: SOS timer for workers */
+ struct list_head idle_list; /* L: list of idle workers */
+ struct delayed_work idle_reaper_work; /* L: worker idle timeout */
+ struct timer_list mayday_timer; /* L: SOS timer for workers */

/* a workers is either on busy_hash or idle_list, or the manager */
DECLARE_HASHTABLE(busy_hash, BUSY_WORKER_HASH_ORDER);
@@ -1806,8 +1806,10 @@ static void worker_enter_idle(struct worker *worker)
/* idle_list is LIFO */
list_add(&worker->entry, &pool->idle_list);

- if (too_many_workers(pool) && !timer_pending(&pool->idle_timer))
- mod_timer(&pool->idle_timer, jiffies + IDLE_WORKER_TIMEOUT);
+ if (too_many_workers(pool) && !delayed_work_pending(&pool->idle_reaper_work))
+ mod_delayed_work(system_unbound_wq,
+ &pool->idle_reaper_work,
+ IDLE_WORKER_TIMEOUT);

/* Sanity check nr_running. */
WARN_ON_ONCE(pool->nr_workers == pool->nr_idle && pool->nr_running);
@@ -1972,9 +1974,72 @@ static struct worker *create_worker(struct worker_pool *pool)
return NULL;
}

+static void unbind_worker(struct worker *worker)
+{
+ lockdep_assert_held(&wq_pool_mutex);
+
+ kthread_set_per_cpu(worker->task, -1);
+ if (cpumask_intersects(wq_unbound_cpumask, cpu_active_mask))
+ WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, wq_unbound_cpumask) < 0);
+ else
+ WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, cpu_possible_mask) < 0);
+}
+
+static void rebind_worker(struct worker *worker, struct worker_pool *pool)
+{
+ kthread_set_per_cpu(worker->task, pool->cpu);
+ WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask) < 0);
+}
+
+static void reap_workers(struct list_head *reaplist)
+{
+ struct worker *worker, *tmp;
+
+ list_for_each_entry_safe(worker, tmp, reaplist, 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 destroy_worker() 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.
+ */
+ WRITE_ONCE(worker->reaped, true);
+ wake_up_process(worker->task);
+ }
+}
+
+/*
+ * Unlikely as it may be, a worker could wake after destroy_worker() has
+ * happened but before reap_workers(). WORKER_DIE would be set in worker->flags,
+ * so it would be able to kfree(worker) and head out to do_exit().
+ *
+ * Rather than make the reaper wait for each to-be-reaped kworker to exit and
+ * kfree(worker) itself, make the kworkers (which have nothing to do but go
+ * do_exit() anyway) wait for the reaper to be done with them.
+ */
+static void worker_wait_reaped(struct worker *worker)
+{
+ WARN_ON_ONCE(current != worker->task);
+
+ for (;;) {
+ set_current_state(TASK_INTERRUPTIBLE);
+ if (READ_ONCE(worker->reaped))
+ break;
+ schedule();
+ }
+ __set_current_state(TASK_RUNNING);
+}
+
/**
* destroy_worker - destroy a workqueue worker
* @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.
@@ -1982,7 +2047,7 @@ static struct worker *create_worker(struct worker_pool *pool)
* CONTEXT:
* raw_spin_lock_irq(pool->lock).
*/
-static void destroy_worker(struct worker *worker)
+static void destroy_worker(struct worker *worker, struct list_head *list)
{
struct worker_pool *pool = worker->pool;

@@ -1997,34 +2062,64 @@ 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);
}

-static void idle_worker_timeout(struct timer_list *t)
+/**
+ * idle_reaper_fn - reap workers that have been idle for too long.
+ *
+ * Unbinding marked-for-destruction workers requires a sleepable context, as
+ * changing a task's affinity is not an atomic operation, and we don't want
+ * to disturb isolated CPUs IDLE_WORKER_TIMEOUT in the future just for a kworker
+ * to do_exit().
+ *
+ * Percpu kworkers should meet the conditions for the affinity change to not
+ * block (not migration-disabled and not running), but there is no *hard*
+ * guarantee that they are not running when we get here.
+ *
+ * The delayed_work is only ever modified under raw_spin_lock_irq(pool->lock).
+ */
+static void idle_reaper_fn(struct work_struct *work)
{
- struct worker_pool *pool = from_timer(pool, t, idle_timer);
+ struct delayed_work *dwork = to_delayed_work(work);
+ struct worker_pool *pool = container_of(dwork, struct worker_pool, idle_reaper_work);
+ struct list_head reaplist;
+ struct worker *worker;
+
+ INIT_LIST_HEAD(&reaplist);

raw_spin_lock_irq(&pool->lock);

while (too_many_workers(pool)) {
- struct worker *worker;
unsigned long expires;
+ unsigned long now = jiffies;

/* idle_list is kept in LIFO order, check the last one */
worker = list_entry(pool->idle_list.prev, struct worker, entry);
expires = worker->last_active + IDLE_WORKER_TIMEOUT;

- if (time_before(jiffies, expires)) {
- mod_timer(&pool->idle_timer, expires);
+ /*
+ * Careful: queueing a work item from here can and will cause a
+ * self-deadlock when dealing with an unbound pool. However,
+ * here the delay *cannot* be zero and *has* to be in the
+ * future, which works.
+ */
+ if (time_before(now, expires)) {
+ mod_delayed_work(system_unbound_wq,
+ &pool->idle_reaper_work,
+ expires - now);
break;
}

- destroy_worker(worker);
+ destroy_worker(worker, &reaplist);
}
-
raw_spin_unlock_irq(&pool->lock);
+
+ mutex_lock(&wq_pool_mutex);
+ reap_workers(&reaplist);
+ mutex_unlock(&wq_pool_mutex);
}

static void send_mayday(struct work_struct *work)
@@ -2388,6 +2483,9 @@ static int worker_thread(void *__worker)
/* am I supposed to die? */
if (unlikely(worker->flags & WORKER_DIE)) {
raw_spin_unlock_irq(&pool->lock);
+
+ worker_wait_reaped(worker);
+
WARN_ON_ONCE(!list_empty(&worker->entry));
set_pf_worker(false);

@@ -3454,7 +3552,7 @@ static int init_worker_pool(struct worker_pool *pool)
INIT_LIST_HEAD(&pool->idle_list);
hash_init(pool->busy_hash);

- timer_setup(&pool->idle_timer, idle_worker_timeout, TIMER_DEFERRABLE);
+ INIT_DEFERRABLE_WORK(&pool->idle_reaper_work, idle_reaper_fn);

timer_setup(&pool->mayday_timer, pool_mayday_timeout, 0);

@@ -3559,8 +3657,11 @@ 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 reaplist;
struct worker *worker;

+ INIT_LIST_HEAD(&reaplist);
+
lockdep_assert_held(&wq_pool_mutex);

if (--pool->refcnt)
@@ -3588,10 +3689,12 @@ static void put_unbound_pool(struct worker_pool *pool)
pool->flags |= POOL_MANAGER_ACTIVE;

while ((worker = first_idle_worker(pool)))
- destroy_worker(worker);
+ destroy_worker(worker, &reaplist);
WARN_ON(pool->nr_workers || pool->nr_idle);
raw_spin_unlock_irq(&pool->lock);

+ reap_workers(&reaplist);
+
mutex_lock(&wq_pool_attach_mutex);
if (!list_empty(&pool->workers))
pool->detach_completion = &detach_completion;
@@ -3601,7 +3704,7 @@ static void put_unbound_pool(struct worker_pool *pool)
wait_for_completion(pool->detach_completion);

/* shut down the timers */
- del_timer_sync(&pool->idle_timer);
+ cancel_delayed_work_sync(&pool->idle_reaper_work);
del_timer_sync(&pool->mayday_timer);

/* RCU protected to allow dereferences from get_work_pool() */
@@ -4999,13 +5102,8 @@ static void unbind_workers(int cpu)

raw_spin_unlock_irq(&pool->lock);

- for_each_pool_worker(worker, pool) {
- kthread_set_per_cpu(worker->task, -1);
- if (cpumask_intersects(wq_unbound_cpumask, cpu_active_mask))
- WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, wq_unbound_cpumask) < 0);
- else
- WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, cpu_possible_mask) < 0);
- }
+ for_each_pool_worker(worker, pool)
+ unbind_worker(worker);

mutex_unlock(&wq_pool_attach_mutex);
}
@@ -5030,11 +5128,8 @@ static void rebind_workers(struct worker_pool *pool)
* of all workers first and then clear UNBOUND. As we're called
* from CPU_ONLINE, the following shouldn't fail.
*/
- for_each_pool_worker(worker, pool) {
- kthread_set_per_cpu(worker->task, pool->cpu);
- WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
- pool->attrs->cpumask) < 0);
- }
+ for_each_pool_worker(worker, pool)
+ rebind_worker(worker, pool);

raw_spin_lock_irq(&pool->lock);

diff --git a/kernel/workqueue_internal.h b/kernel/workqueue_internal.h
index e00b1204a8e9..a3d60e10a76f 100644
--- a/kernel/workqueue_internal.h
+++ b/kernel/workqueue_internal.h
@@ -46,6 +46,7 @@ struct worker {
unsigned int flags; /* X: flags */
int id; /* I: worker id */
int sleeping; /* None */
+ int reaped; /* None */

/*
* Opaque string set with work_set_desc(). Printed out with task
--
2.31.1
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