[RFC PATCH] sched: Invoke io_wq_worker_sleeping() with enabled preemption

[Sebastian Andrzej Siewior]

During a context switch the scheduler invokes wq_worker_sleeping() with
disabled preemption. Disabling preemption is needed because it protects
access to `worker->sleeping'. As an optimisation it avoids invoking
schedule() within the schedule path as part of possible wake up (thus
preempt_enable_no_resched() afterwards).

The io-wq has been added to the mix in the same section with disabled
preemption. This breaks on PREEMPT_RT because io_wq_worker_sleeping()
acquires a spinlock_t. Also within the schedule() the spinlock_t must be
acquired after tsk_is_pi_blocked() otherwise it will block on the sleeping lock
again while scheduling out.

While playing with `io_uring-bench' I didn't notice a significant
latency spike after converting io_wqe::lock to a raw_spinlock_t. The
latency was more or less the same.

I don't see a significant reason why this lock should become a
raw_spinlock_t therefore I suggest to move it after the
tsk_is_pi_blocked() check.
The io_worker::flags are usually modified under the lock except in the
scheduler path. Ideally the lock is always acquired since the
IO_WORKER_F_UP flag is set early in the startup and IO_WORKER_F_RUNNING
should be set unless the task loops within schedule(). I *think* ::flags
requires the same protection like workqueue's ::sleeping and therefore I
move the check within the locked section.

Any feedback on this vs raw_spinlock_t?

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 fs/io-wq.c          |  8 ++++----
 kernel/sched/core.c | 10 +++++-----
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index e92c4724480ca..a7e07b3ac5b95 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -623,15 +623,15 @@ void io_wq_worker_sleeping(struct task_struct *tsk)
 	struct io_worker *worker = kthread_data(tsk);
 	struct io_wqe *wqe = worker->wqe;
 
+	spin_lock_irq(&wqe->lock);
 	if (!(worker->flags & IO_WORKER_F_UP))
-		return;
+		goto out;
 	if (!(worker->flags & IO_WORKER_F_RUNNING))
-		return;
+		goto out;
 
 	worker->flags &= ~IO_WORKER_F_RUNNING;
-
-	spin_lock_irq(&wqe->lock);
 	io_wqe_dec_running(wqe, worker);
+out:
 	spin_unlock_irq(&wqe->lock);
 }
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3bbb60b97c73c..b76c0f27bd95e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4694,18 +4694,18 @@ static inline void sched_submit_work(struct task_struct *tsk)
 	 * in the possible wakeup of a kworker and because wq_worker_sleeping()
 	 * requires it.
 	 */
-	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
+	if (tsk->flags & PF_WQ_WORKER) {
 		preempt_disable();
-		if (tsk->flags & PF_WQ_WORKER)
-			wq_worker_sleeping(tsk);
-		else
-			io_wq_worker_sleeping(tsk);
+		wq_worker_sleeping(tsk);
 		preempt_enable_no_resched();
 	}
 
 	if (tsk_is_pi_blocked(tsk))
 		return;
 
+	if (tsk->flags & PF_IO_WORKER)
+		io_wq_worker_sleeping(tsk);
+
 	/*
 	 * If we are going to sleep and we have plugged IO queued,
 	 * make sure to submit it to avoid deadlocks.
-- 
2.28.0