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@xxxxxxxxx>
---
v3:
Prevented the flush the easy way: workque is not flused when low latency
flag is set. This means that drivers that use the tty_schedule_flip()
functions instead of the tty_flip_buffer_push() should never have the
low_latency flag set, otherwise the workqueue won't be flushed. Added a
WARN_ON() to detect those drivers.
For PREEMPT_RT systems: this optimization will only work when the user has
actually enabled the ASYNC_LOW_LATENCY option for the serial driver, otherwise
the workqueue will still be flushed although it is not used since the PREMPT_RT
patch makes the tty_flip_buffer_push() function to always bypass the workqueue.
drivers/tty/tty_buffer.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 6146e8b..b952de1 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
*/
@@ -349,6 +351,7 @@ EXPORT_SYMBOL(tty_insert_flip_string_flags);
void tty_schedule_flip(struct tty_struct *tty)
{
unsigned long flags;
+ WARN_ON(tty->low_latency);
spin_lock_irqsave(&tty->buf.lock, flags);
if (tty->buf.tail != NULL)
tty->buf.tail->commit = tty->buf.tail->used;
@@ -514,7 +517,8 @@ 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);
}
/**
--
1.7.9.5
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