linux-kernel - [PATCH [RT] 06/14] optimize rt lock wakeup

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Date:	Thu, 21 Feb 2008 10:26:56 -0500
From:	Gregory Haskins <ghaskins@...ell.com>
To:	mingo@...e.hu, a.p.zijlstra@...llo.nl, tglx@...utronix.de,
	rostedt@...dmis.org, linux-rt-users@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, bill.huey@...il.com,
	kevin@...man.org, cminyard@...sta.com, dsingleton@...sta.com,
	dwalker@...sta.com, npiggin@...e.de, dsaxena@...xity.net,
	ak@...e.de, gregkh@...e.de, sdietrich@...ell.com,
	pmorreale@...ell.com, mkohari@...ell.com, ghaskins@...ell.com
Subject: [PATCH [RT] 06/14] optimize rt lock wakeup

It is redundant to wake the grantee task if it is already running

Credit goes to Peter for the general idea.

Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
Signed-off-by: Peter Morreale <pmorreale@...ell.com>
---

 kernel/rtmutex.c |   23 ++++++++++++++++++-----
 1 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 15fc6e6..cb27b08 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -511,6 +511,24 @@ 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 {
+		smp_mb();
+		/*
+		 * This may appear to be a race, but the barriers close the
+		 * window.
+		 */
+		if ((pendowner->state != TASK_RUNNING)
+		    && (pendowner->state != TASK_RUNNING_MUTEX))
+			wake_up_process_mutex(pendowner);
+	}
+
 	rt_mutex_set_owner(lock, pendowner, RT_MUTEX_OWNER_PENDING);
 
 	spin_unlock(&current->pi_lock);
@@ -537,11 +555,6 @@ 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);
 }
 
 /*

--
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