From 5ede52538ee2b2202d9dff5b06c33bfde421e6e4 Mon Sep 17 00:00:00 2001 From: Peter Hurley Date: Wed, 24 Jul 2013 08:29:57 -0400 Subject: [PATCH] tty: Remove extra wakeup from pty write() path Acquiring the write_wait queue spin lock now accounts for the largest slice of cpu time on the tty write path. Two factors contribute to this situation; a overly-pessimistic line discipline write loop which _always_ sets up a wait loop even if i/o will immediately succeed, and on ptys, a wakeup storm from reads and writes. Writer wakeup does not need to be performed by the pty driver. Firstly, since the actual i/o is performed within the write, the line discipline write loop will continue while space remains in the flip buffers. Secondly, when space becomes avail in the line discipline receive buffer (and thus also in the flip buffers), the pty unthrottle re-wakes the writer (non-flow-controlled line disciplines unconditionally unthrottle the driver when data is received). Thus, existing in-kernel i/o is guaranteed to advance. Finally, writer wakeup occurs at the conclusion of the line discipline write (in tty_write_unlock()). This guarantees that any user-space write waiters are woken to continue additional i/o. Signed-off-by: Peter Hurley Signed-off-by: Greg Kroah-Hartman --- drivers/tty/pty.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c index b38a28bd9511..b940127ba1c8 100644 --- a/drivers/tty/pty.c +++ b/drivers/tty/pty.c @@ -121,10 +121,8 @@ static int pty_write(struct tty_struct *tty, const unsigned char *buf, int c) /* Stuff the data into the input queue of the other end */ c = tty_insert_flip_string(to->port, buf, c); /* And shovel */ - if (c) { + if (c) tty_flip_buffer_push(to->port); - tty_wakeup(tty); - } } return c; }