io-wq uses an idle timeout to shrink the pool, but keeps the last worker
around indefinitely to avoid churn.

For tasks that used io_uring for file I/O and then stop using io_uring,
this can leave an iou-wrk-* thread behind even after all io_uring instances
are gone. This is unnecessary overhead and also gets in the way of process
checkpoint/restore.

Add an exit-on-idle mode that makes all io-wq workers exit as soon as they
become idle, and provide io_wq_set_exit_on_idle() to toggle it.

Signed-off-by: Li Chen <me@linux.beauty>
---
 io_uring/io-wq.c | 31 +++++++++++++++++++++++++++++++
 io_uring/io-wq.h |  1 +
 2 files changed, 32 insertions(+)

diff --git a/io_uring/io-wq.c b/io_uring/io-wq.c
index 5d0928f37471..97e7eb847c6e 100644
--- a/io_uring/io-wq.c
+++ b/io_uring/io-wq.c
@@ -35,6 +35,7 @@ enum {
 
 enum {
 	IO_WQ_BIT_EXIT		= 0,	/* wq exiting */
+	IO_WQ_BIT_EXIT_ON_IDLE	= 1,	/* allow all workers to exit on idle */
 };
 
 enum {
@@ -655,6 +656,18 @@ static int io_wq_worker(void *data)
 			io_worker_handle_work(acct, worker);
 
 		raw_spin_lock(&wq->lock);
+		/*
+		 * If wq is marked idle-exit, drop this worker as soon as it
+		 * becomes idle. This is used to avoid keeping io-wq worker
+		 * threads around for tasks that no longer have any active
+		 * io_uring instances.
+		 */
+		if (test_bit(IO_WQ_BIT_EXIT_ON_IDLE, &wq->state)) {
+			acct->nr_workers--;
+			raw_spin_unlock(&wq->lock);
+			__set_current_state(TASK_RUNNING);
+			break;
+		}
 		/*
 		 * Last sleep timed out. Exit if we're not the last worker,
 		 * or if someone modified our affinity.
@@ -894,6 +907,24 @@ static bool io_wq_worker_wake(struct io_worker *worker, void *data)
 	return false;
 }
 
+void io_wq_set_exit_on_idle(struct io_wq *wq, bool enable)
+{
+	if (!wq->task)
+		return;
+
+	if (!enable) {
+		clear_bit(IO_WQ_BIT_EXIT_ON_IDLE, &wq->state);
+		return;
+	}
+
+	if (test_and_set_bit(IO_WQ_BIT_EXIT_ON_IDLE, &wq->state))
+		return;
+
+	rcu_read_lock();
+	io_wq_for_each_worker(wq, io_wq_worker_wake, NULL);
+	rcu_read_unlock();
+}
+
 static void io_run_cancel(struct io_wq_work *work, struct io_wq *wq)
 {
 	do {
diff --git a/io_uring/io-wq.h b/io_uring/io-wq.h
index b3b004a7b625..f7f17a23693e 100644
--- a/io_uring/io-wq.h
+++ b/io_uring/io-wq.h
@@ -46,6 +46,7 @@ struct io_wq_data {
 struct io_wq *io_wq_create(unsigned bounded, struct io_wq_data *data);
 void io_wq_exit_start(struct io_wq *wq);
 void io_wq_put_and_exit(struct io_wq *wq);
+void io_wq_set_exit_on_idle(struct io_wq *wq, bool enable);
 
 void io_wq_enqueue(struct io_wq *wq, struct io_wq_work *work);
 void io_wq_hash_work(struct io_wq_work *work, void *val);
-- 
2.52.0