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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>
This commit is contained in:
Ivo Sieben 2012-09-27 14:02:05 +02:00 committed by Greg Kroah-Hartman
parent daee779718
commit cee4ad1ed9
1 changed files with 4 additions and 1 deletions
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5
drivers/tty/tty_buffer.c
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@ -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);
}
/**