From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754692Ab3FOO3c (ORCPT ); Sat, 15 Jun 2013 10:29:32 -0400 Received: from mailout01.c08.mtsvc.net ([205.186.168.189]:40633 "EHLO mailout01.c08.mtsvc.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754226Ab3FOO14 (ORCPT ); Sat, 15 Jun 2013 10:27:56 -0400 From: Peter Hurley To: Greg Kroah-Hartman Cc: linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, Jiri Slaby , Peter Hurley Subject: [PATCH v2 20/20] tty: Remove extra wakeup from pty write() path Date: Sat, 15 Jun 2013 10:21:36 -0400 Message-Id: <1371306096-5571-21-git-send-email-peter@hurleysoftware.com> X-Mailer: git-send-email 1.8.1.2 In-Reply-To: <1371306096-5571-1-git-send-email-peter@hurleysoftware.com> References: <1371305069-5366-1-git-send-email-peter@hurleysoftware.com> <1371306096-5571-1-git-send-email-peter@hurleysoftware.com> X-Authenticated-User: 125194 peter@hurleysoftware.com Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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 0634dd9..b9bc5be 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; } -- 1.8.1.2