From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933712AbYETPWX (ORCPT ); Tue, 20 May 2008 11:22:23 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932883AbYETPVr (ORCPT ); Tue, 20 May 2008 11:21:47 -0400 Received: from 66-189-124-104.dhcp.oxfr.ma.charter.com ([66.189.124.104]:57727 "EHLO novell1.haskins.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932620AbYETPVp (ORCPT ); Tue, 20 May 2008 11:21:45 -0400 From: Gregory Haskins Subject: [PATCH 1/5] optimize rt lock wakeup To: mingo@elte.hu, tglx@linutronix.de, rostedt@goodmis.org, linux-rt-users@vger.kernel.org Cc: linux-kernel@vger.kernel.org, sdietrich@novell.com, pmorreale@novell.com, mkohari@novell.com, ghaskins@novell.com Date: Tue, 20 May 2008 10:49:15 -0400 Message-ID: <20080520144915.15992.57013.stgit@novell1.haskins.net> In-Reply-To: <20080520143954.15992.85900.stgit@novell1.haskins.net> References: <20080520143954.15992.85900.stgit@novell1.haskins.net> User-Agent: StGIT/0.12.1 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org It is redundant to wake the grantee task if it is already running, and the call to wake_up_process is relatively expensive. If we can safely skip it we can measurably improve the performance of the adaptive-locks. Credit goes to Peter Morreale for the general idea. Signed-off-by: Gregory Haskins Signed-off-by: Peter Morreale --- kernel/rtmutex.c | 45 ++++++++++++++++++++++++++++++++++++++++----- 1 files changed, 40 insertions(+), 5 deletions(-) diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c index 6c1debb..8ae9de3 100644 --- a/kernel/rtmutex.c +++ b/kernel/rtmutex.c @@ -522,6 +522,41 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate) pendowner = waiter->task; waiter->task = NULL; + /* + * Do the wakeup before the ownership change to give any spinning + * waiter grantees a headstart over the other threads that will + * trigger once owner changes. + */ + if (!savestate) + wake_up_process(pendowner); + else { + /* + * We can skip the actual (expensive) wakeup if the + * waiter is already running, but we have to be careful + * of race conditions because they may be about to sleep. + * + * The waiter-side protocol has the following pattern: + * 1: Set state != RUNNING + * 2: Conditionally sleep if waiter->task != NULL; + * + * And the owner-side has the following: + * A: Set waiter->task = NULL + * B: Conditionally wake if the state != RUNNING + * + * As long as we ensure 1->2 order, and A->B order, we + * will never miss a wakeup. + * + * Therefore, this barrier ensures that waiter->task = NULL + * is visible before we test the pendowner->state. The + * corresponding barrier is in the sleep logic. + */ + smp_mb(); + + /* If !RUNNING && !RUNNING_MUTEX */ + if (pendowner->state & ~TASK_RUNNING_MUTEX) + wake_up_process_mutex(pendowner); + } + rt_mutex_set_owner(lock, pendowner, RT_MUTEX_OWNER_PENDING); spin_unlock(¤t->pi_lock); @@ -548,11 +583,6 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate) plist_add(&next->pi_list_entry, &pendowner->pi_waiters); } spin_unlock(&pendowner->pi_lock); - - if (savestate) - wake_up_process_mutex(pendowner); - else - wake_up_process(pendowner); } /* @@ -803,6 +833,11 @@ rt_spin_lock_slowlock(struct rt_mutex *lock) if (adaptive_wait(&waiter, orig_owner)) { update_current(TASK_UNINTERRUPTIBLE, &saved_state); + /* + * The xchg() in update_current() is an implicit + * barrier which we rely upon to ensure current->state + * is visible before we test waiter.task. + */ if (waiter.task) schedule_rt_mutex(lock); }