In case of PREEMPT_RT or when low_latency flag is set by the serial driver
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 check & wait for the workqueue to finish.
This prevents unnecessary spin lock/unlock on the workqueue spin lock that can
cause additional scheduling overhead on a PREEMPT_RT system. On a 240 MHz
AT91SAM9261 processor setup this fixes about 100us of scheduling overhead on the
TTY read call.
Signed-off-by: Ivo Sieben <meltedpianoman@xxxxxxxxx>
---
drivers/tty/tty_buffer.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 5cfa548..b8e90b8 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -492,7 +492,16 @@ static void flush_to_ldisc(struct work_struct *work)
*/
void tty_flush_to_ldisc(struct tty_struct *tty)
{
- flush_work(&tty->buf.work);
+ /*
+ * Only in case a workqueue is actually used for copying data to the
+ * line discipline, we'll have to wait for the workqueue to finish. In
+ * other cases this prevents us from unnecessary blocking by the
+ * workqueue spin lock.
+ */
+#ifndef CONFIG_PREEMPT_RT_FULL
+ if (!tty->low_latency)
+ flush_work(&tty->buf.work);
+#endif
}
/**
--
1.7.9.5
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