io_uring/sqpoll: wait on sqd->wait for thread parking

io_sqd_handle_event() just does a mutex unlock/lock dance when it's
supposed to park, somewhat relying on full ordering with the thread
trying to park it which does a similar unlock/lock dance on sqd->lock.
However, with adaptive spinning on mutexes, this can waste an awful
lot of time. Normally this isn't very noticeable, as parking and
unparking the thread isn't a common (or fast path) occurence. However,
in testing ring resizing, it's testing exactly that, as each resize
will require the SQPOLL to safely park and unpark.

Have io_sq_thread_park() explicitly wait on sqd->park_pending being
zero before attempting to grab the sqd->lock again.

In a resize test, this brings the runtime of SQPOLL down from about
60 seconds to a few seconds, just like the !SQPOLL tests. And saves
a ton of spinning time on the mutex, on both sides.

Signed-off-by: Jens Axboe <[email protected]>

Author: axboe

io_uring/sqpoll.c

@@ -40,6 +40,7 @@ void io_sq_thread_unpark(struct io_sq_data *sqd)
 	if (atomic_dec_return(&sqd->park_pending))
 		set_bit(IO_SQ_THREAD_SHOULD_PARK, &sqd->state);
 	mutex_unlock(&sqd->lock);
+	wake_up(&sqd->wait);
 }
 
 void io_sq_thread_park(struct io_sq_data *sqd)
@@ -215,7 +216,7 @@ static bool io_sqd_handle_event(struct io_sq_data *sqd)
 		mutex_unlock(&sqd->lock);
 		if (signal_pending(current))
 			did_sig = get_signal(&ksig);
-		cond_resched();
+		wait_event(sqd->wait, !atomic_read(&sqd->park_pending));
 		mutex_lock(&sqd->lock);
 		sqd->sq_cpu = raw_smp_processor_id();
 	}