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>
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 91e326f..8b00f6a 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -342,6 +342,8 @@
* 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 @@
*/
void tty_flush_to_ldisc(struct tty_struct *tty)
{
- flush_work(&tty->buf.work);
+ if (!tty->low_latency)
+ flush_work(&tty->buf.work);
}
/**