io_uring/io-wq: add exit-on-idle state

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 state 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>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: FirstLoveLife

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 {
@@ -707,9 +708,13 @@ static int io_wq_worker(void *data)
 		raw_spin_lock(&acct->workers_lock);
 		/*
 		 * Last sleep timed out. Exit if we're not the last worker,
-		 * or if someone modified our affinity.
+		 * or if someone modified our affinity. If wq is marked
+		 * idle-exit, drop the worker as well. This is used to avoid
+		 * keeping io-wq workers around for tasks that no longer have
+		 * any active io_uring instances.
 		 */
-		if (last_timeout && (exit_mask || acct->nr_workers > 1)) {
+		if ((last_timeout && (exit_mask || acct->nr_workers > 1)) ||
+		    test_bit(IO_WQ_BIT_EXIT_ON_IDLE, &wq->state)) {
 			acct->nr_workers--;
 			raw_spin_unlock(&acct->workers_lock);
 			__set_current_state(TASK_RUNNING);
@@ -967,6 +972,24 @@ 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 {

io_uring/io-wq.h

@@ -41,6 +41,7 @@ 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);