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
