tty: prevent unnecessary work queue lock checking on flip buffer copy

When low_latency flag is set the TTY receive flip buffer is copied to the
line discipline directly instead of using a work queue in the background.
Therefor only in case a workqueue is actually used for copying data to the
line discipline we'll have to flush the workqueue.

This prevents unnecessary spin lock/unlock on the workqueue spin lock that
can cause additional scheduling overhead on a PREEMPT_RT system. On a 200
MHz AT91SAM9261 processor setup this fixes about 100us of scheduling
overhead on the TTY read call.

Signed-off-by: Ivo Sieben <meltedpianoman@gmail.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 91e326ffe7db..8b00f6a34a7d 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -342,6 +342,8 @@ EXPORT_SYMBOL(tty_insert_flip_string_flags);
  *      Takes any pending buffers and transfers their ownership to the
  *      ldisc side of the queue. It then schedules those characters for
  *      processing by the line discipline.
+ *      Note that this function can only be used when the low_latency flag
+ *      is unset. Otherwise the workqueue won't be flushed.
  *
  *      Locking: Takes tty->buf.lock
  */
@@ -514,7 +516,8 @@ static void flush_to_ldisc(struct work_struct *work)
  */
 void tty_flush_to_ldisc(struct tty_struct *tty)
 {
-        flush_work(&tty->buf.work);
+        if (!tty->low_latency)
+                flush_work(&tty->buf.work);
 }
 
 /**