[PATCH v6 3/4] workqueue: Convert the idle_timer to a timer + work_struct

From: Valentin Schneider
Date: Mon Nov 28 2022 - 13:34:29 EST


A later patch will require a sleepable context in the idle worker timeout
function. Converting worker_pool.idle_timer to a delayed_work gives us just
that, however this would imply turning all idle_timer expiries into
scheduler events (waking up a worker to handle the dwork).

Instead, implement a "custom dwork" where the timer callback does some
extra checks before queuing the associated work.

No change in functionality intended.

Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
---
kernel/workqueue.c | 51 +++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 48 insertions(+), 3 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 8185a42848c50..c8b1466a9c070 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -169,7 +169,9 @@ struct worker_pool {

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 work_struct idle_cull_work; /* L: worker idle cleanup */
+
+ 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);
@@ -1806,7 +1808,9 @@ 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))
+ if (too_many_workers(pool) &&
+ !timer_pending(&pool->idle_timer) &&
+ !work_pending(&pool->idle_cull_work))
mod_timer(&pool->idle_timer, jiffies + IDLE_WORKER_TIMEOUT);

/* Sanity check nr_running. */
@@ -2019,17 +2023,56 @@ static void destroy_worker(struct worker *worker)
wake_up_process(worker->task);
}

+/*
+ * idle_worker_timeout - check if some idle workers can now be deleted.
+ *
+ * The timer is armed in worker_enter_idle(). Note that it isn't disarmed in
+ * worker_leave_idle(), as a worker flicking between idle and active while its
+ * pool is at the too_many_workers() tipping point would cause too much timer
+ * housekeeping overhead. Since IDLE_WORKER_TIMEOUT is long enough, we just let
+ * it expire and re-evaluate things from there.
+ */
static void idle_worker_timeout(struct timer_list *t)
{
struct worker_pool *pool = from_timer(pool, t, idle_timer);
+ bool do_cull = false;
+
+ if (work_pending(&pool->idle_cull_work))
+ return;

raw_spin_lock_irq(&pool->lock);

- while (too_many_workers(pool)) {
+ if (too_many_workers(pool)) {
struct worker *worker;
unsigned long expires;

/* 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;
+ do_cull = !time_before(jiffies, expires);
+
+ if (!do_cull)
+ mod_timer(&pool->idle_timer, expires);
+ }
+ raw_spin_unlock_irq(&pool->lock);
+
+ if (do_cull)
+ queue_work(system_unbound_wq, &pool->idle_cull_work);
+}
+
+/*
+ * idle_cull_fn - cull workers that have been idle for too long.
+ */
+static void idle_cull_fn(struct work_struct *work)
+{
+ struct worker_pool *pool = container_of(work, struct worker_pool, idle_cull_work);
+
+ raw_spin_lock_irq(&pool->lock);
+
+ while (too_many_workers(pool)) {
+ struct worker *worker;
+ unsigned long expires;
+
worker = list_entry(pool->idle_list.prev, struct worker, entry);
expires = worker->last_active + IDLE_WORKER_TIMEOUT;

@@ -3479,6 +3522,7 @@ static int init_worker_pool(struct worker_pool *pool)
hash_init(pool->busy_hash);

timer_setup(&pool->idle_timer, idle_worker_timeout, TIMER_DEFERRABLE);
+ INIT_WORK(&pool->idle_cull_work, idle_cull_fn);

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

@@ -3626,6 +3670,7 @@ static void put_unbound_pool(struct worker_pool *pool)

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

/* RCU protected to allow dereferences from get_work_pool() */
--
2.31.1