From: Yu Kuai <redacted>

Currently, wake_up() is called unconditionally in fast path such as
raid10_make_request(), which will cause lock contention under high
concurrency:

raid10_make_request
  wake_up
   __wake_up_common_lock
    spin_lock_irqsave

Improve performance by only call wake_up() if waitqueue is not empty
in allow_barrier() and raid10_make_request().

Signed-off-by: Yu Kuai <redacted>
---
  drivers/md/raid10.c | 10 ++++++++--
  1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index df435d693637..cf452c25e1e5 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c

@@ -274,6 +274,12 @@ static void put_buf(struct r10bio *r10_bio)
  	lower_barrier(conf);
  }
  
+static void wake_up_barrier(struct r10conf *conf)
+{
+	if (wq_has_sleeper(&conf->wait_barrier))
+		wake_up(&conf->wait_barrier);
+}
+
  static void reschedule_retry(struct r10bio *r10_bio)
  {
  	unsigned long flags;

@@ -1015,7 +1021,7 @@ static void allow_barrier(struct r10conf *conf)
  {
  	if ((atomic_dec_and_test(&conf->nr_pending)) ||
  			(conf->array_freeze_pending))
-		wake_up(&conf->wait_barrier);
+		wake_up_barrier(conf);
  }
  
  static void freeze_array(struct r10conf *conf, int extra)

@@ -1891,7 +1897,7 @@ static bool raid10_make_request(struct mddev *mddev, struct bio *bio)
  	__make_request(mddev, bio, sectors);
  
  	/* In case raid10d snuck in to freeze_array */
-	wake_up(&conf->wait_barrier);
+	wake_up_barrier(conf);
  	return true;
  }