tty: stop using "delayed_work" in the tty layer

Using delayed-work for tty flip buffers ends up causing us to wait for
the next tick to complete some actions.  That's usually not all that
noticeable, but for certain latency-critical workloads it ends up being
totally unacceptable.

As an extreme case of this, passing a token back-and-forth over a pty
will take two ticks per iteration, so even just a thousand iterations
will take 8 seconds assuming a common 250Hz configuration.

Avoiding the whole delayed work issue brings that ping-pong test-case
down to 0.009s on my machine.

In more practical terms, this latency has been a performance problem for
things like dive computer simulators (simulating the serial interface
using the ptys) and for other environments (Alan mentions a CP/M emulator).

Reported-by: Jef Driesen <jefdriesen@telenet.be>
Acked-by: Greg KH <gregkh@suse.de>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 7 deletions

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index d8210ca00720..b9451219528b 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -322,7 +322,7 @@ void tty_schedule_flip(struct tty_struct *tty)
         if (tty->buf.tail != NULL)
                 tty->buf.tail->commit = tty->buf.tail->used;
         spin_unlock_irqrestore(&tty->buf.lock, flags);
-        schedule_delayed_work(&tty->buf.work, 1);
+        schedule_work(&tty->buf.work);
 }
 EXPORT_SYMBOL(tty_schedule_flip);
 
@@ -402,7 +402,7 @@ EXPORT_SYMBOL_GPL(tty_prepare_flip_string_flags);
 static void flush_to_ldisc(struct work_struct *work)
 {
         struct tty_struct *tty =
-                container_of(work, struct tty_struct, buf.work.work);
+                container_of(work, struct tty_struct, buf.work);
         unsigned long   flags;
         struct tty_ldisc *disc;
 
@@ -443,7 +443,7 @@ static void flush_to_ldisc(struct work_struct *work)
                         if (test_bit(TTY_FLUSHPENDING, &tty->flags))
                                 break;
                         if (!tty->receive_room || seen_tail) {
-                                schedule_delayed_work(&tty->buf.work, 1);
+                                schedule_work(&tty->buf.work);
                                 break;
                         }
                         if (count > tty->receive_room)
@@ -481,7 +481,7 @@ static void flush_to_ldisc(struct work_struct *work)
  */
 void tty_flush_to_ldisc(struct tty_struct *tty)
 {
-        flush_delayed_work(&tty->buf.work);
+        flush_work(&tty->buf.work);
 }
 
 /**
@@ -506,9 +506,9 @@ void tty_flip_buffer_push(struct tty_struct *tty)
         spin_unlock_irqrestore(&tty->buf.lock, flags);
 
         if (tty->low_latency)
-                flush_to_ldisc(&tty->buf.work.work);
+                flush_to_ldisc(&tty->buf.work);
         else
-                schedule_delayed_work(&tty->buf.work, 1);
+                schedule_work(&tty->buf.work);
 }
 EXPORT_SYMBOL(tty_flip_buffer_push);
 
@@ -529,6 +529,6 @@ void tty_buffer_init(struct tty_struct *tty)
         tty->buf.tail = NULL;
         tty->buf.free = NULL;
         tty->buf.memory_used = 0;
-        INIT_DELAYED_WORK(&tty->buf.work, flush_to_ldisc);
+        INIT_WORK(&tty->buf.work, flush_to_ldisc);
 }