tty: Only wakeup the line discipline idle queue when queue is active

Before waking up the tty line discipline idle queue first check if the queue is
active (non empty). This prevents unnecessary entering the critical section in
the wake_up() function and therefore avoid needless scheduling overhead on a
PREEMPT_RT system caused by two processes being in the same critical section.

Signed-off-by: Ivo Sieben <meltedpianoman@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index c5782294e53261e701f13292b5732c231cfb9d14..e96d1876bd629e9abe6e62be8218c6c3f85385aa 100644 (file)
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -64,7 +64,9 @@ static void put_ldisc(struct tty_ldisc *ld)
                return;
        }
        raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
-       wake_up(&ld->wq_idle);
+
+       if (waitqueue_active(&ld->wq_idle))
+               wake_up(&ld->wq_idle);
 }
 
 /**