[PATCH v5 4/5] workqueue: Convert the idle_timer to a timer + work_struct

[Valentin Schneider]

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.

The new worker_pool.idle_cull_list is made ____cacheline_aligned to prevent
it from sitting over two cachelines.

Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
---
 kernel/workqueue.c | 68 +++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 64 insertions(+), 4 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 4fc8085f3fe17..b744288c58a4b 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -169,7 +169,12 @@ 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 list_head	idle_cull_list  /* L: list of idle workers to cull */
+	____cacheline_aligned;
+	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);
@@ -1812,7 +1817,9 @@ static void worker_enter_idle(struct worker *worker)
 	/* idle_list is LIFO */
 	list_add(&worker->entry, &pool->idle_list);
 
-	if (worker_cull_count(pool) && !timer_pending(&pool->idle_timer))
+	if (worker_cull_count(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. */
@@ -2025,13 +2032,27 @@ 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 worker_cull_count() 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);
+	unsigned int max_cull_cnt, cull_cnt;
+
+	if (work_pending(&pool->idle_cull_work))
+		return;
 
 	raw_spin_lock_irq(&pool->lock);
 
-	while (worker_cull_count(pool)) {
+	max_cull_cnt = worker_cull_count(pool);
+	for (cull_cnt = 0; cull_cnt < max_cull_cnt; cull_cnt++) {
 		struct worker *worker;
 		unsigned long expires;
 
@@ -2039,12 +2060,48 @@ static void idle_worker_timeout(struct timer_list *t)
 		worker = list_entry(pool->idle_list.prev, struct worker, entry);
 		expires = worker->last_active + IDLE_WORKER_TIMEOUT;
 
+		/* All remaining entries will be younger than this */
 		if (time_before(jiffies, expires)) {
-			mod_timer(&pool->idle_timer, expires);
+			if (!cull_cnt)
+				mod_timer(&pool->idle_timer, expires);
 			break;
 		}
 
+		/*
+		 * Mark the idle worker ripe for culling.
+		 * If a preempted idle worker gets to run before the idle cull
+		 * handles it, it will just pop itself out of that list and
+		 * continue as normal.
+		 */
+		list_move(&worker->entry, &pool->idle_cull_list);
+	}
+	raw_spin_unlock_irq(&pool->lock);
+
+	if (cull_cnt)
+		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);
+	struct worker *worker, *tmp;
+
+	raw_spin_lock_irq(&pool->lock);
+
+	list_for_each_entry_safe(worker, tmp, &pool->idle_cull_list, entry)
 		destroy_worker(worker);
+
+	/* Re-arm the idle timer if necessary */
+	if (pool->nr_idle) {
+		unsigned long expires;
+
+		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);
 	}
 
 	raw_spin_unlock_irq(&pool->lock);
@@ -3482,9 +3539,11 @@ static int init_worker_pool(struct worker_pool *pool)
 	pool->watchdog_ts = jiffies;
 	INIT_LIST_HEAD(&pool->worklist);
 	INIT_LIST_HEAD(&pool->idle_list);
+	INIT_LIST_HEAD(&pool->idle_cull_list);
 	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);
 
@@ -3632,6 +3691,7 @@ 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