Commit cee4ad1e authored Sep 27, 2012 by Ivo Sieben Committed by Greg Kroah-Hartman Oct 22, 2012

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>

parent daee7797

drivers/tty/tty_buffer.c

+4 −1

Original line number	Diff line number	Diff line
		@@ -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
		*/
		@@ -514,6 +516,7 @@ static void flush_to_ldisc(struct work_struct *work)
		*/
		void tty_flush_to_ldisc(struct tty_struct *tty)
		{
		if (!tty->low_latency)
		flush_work(&tty->buf.work);
		}

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