[PATCH v4 3/4] workqueue: Convert the idle_timer to a delayed_work

[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.

One caveat is that we need to be careful about re-queuing the dwork from
its callback function. Lai expressed concerns about overtly violating
documented locking rules, but extra locking is required around delaying the
dwork, else a worker thread adding itself to the idle_list might push the
dwork further back (IDLE_WORKER_TIMEOUT) than the work callback would (next
idle worker expiry).

No change in functionality intended.

Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
---
 kernel/workqueue.c | 41 +++++++++++++++++++++++++++++------------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 8185a42848c5..436b1dbdf9ff 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);
@@ -2019,22 +2021,37 @@ static void destroy_worker(struct worker *worker)
 	wake_up_process(worker->task);
 }
 
-static void idle_worker_timeout(struct timer_list *t)
+/*
+ * idle_reaper_fn - reap workers that have been idle for too long.
+ *
+ * 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);
 
 	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 */
+		/* idle_list is kept in LIFO order, check the oldest entry */
 		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, so the queuing will
+		 * happen in the timer callback.
+		 */
+		if (time_before(now, expires)) {
+			mod_delayed_work(system_unbound_wq,
+					 &pool->idle_reaper_work,
+					 expires - now);
 			break;
 		}
 
@@ -3478,7 +3495,7 @@ 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);
 
@@ -3625,7 +3642,7 @@ 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() */
-- 
2.31.1