linux-kernel - [PATCH] locking/rtmutex: Optimize setting task running after being blocked

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Date:	Sun, 01 Feb 2015 22:16:24 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dave@...olabs.net>
Subject: [PATCH] locking/rtmutex: Optimize setting task running after being
 blocked

We explicitly mark the task running after returning from
a __rt_mutex_slowlock() call, which does the actual sleeping
via wait-wake-trylocking. As such, this patch does two things:

(1) refactors the code so that setting current to TASK_RUNNING
is done by __rt_mutex_slowlock(), and not by the callers. The
downside to this is that it becomes a bit unclear when at what
point we block. As such I've added a comment that the task
blocks when calling __rt_mutex_slowlock() so readers can figure
out when it is running again.

(2) relaxes setting current's state through __set_current_state(),
instead of it's more expensive barrier alternative. There was no
need for the implied barrier as we're obviously not planning on
blocking.

Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
---
 kernel/locking/rtmutex.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 7c98873..3059bc2f 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1130,6 +1130,7 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
 		set_current_state(state);
 	}
 
+	__set_current_state(TASK_RUNNING);
 	return ret;
 }
 
@@ -1188,10 +1189,9 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
 	ret = task_blocks_on_rt_mutex(lock, &waiter, current, chwalk);
 
 	if (likely(!ret))
+		/* sleep on the mutex */
 		ret = __rt_mutex_slowlock(lock, state, timeout, &waiter);
 
-	set_current_state(TASK_RUNNING);
-
 	if (unlikely(ret)) {
 		remove_waiter(lock, &waiter);
 		rt_mutex_handle_deadlock(ret, chwalk, &waiter);
@@ -1626,10 +1626,9 @@ int rt_mutex_finish_proxy_lock(struct rt_mutex *lock,
 
 	set_current_state(TASK_INTERRUPTIBLE);
 
+	/* sleep on the mutex */
 	ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter);
 
-	set_current_state(TASK_RUNNING);
-
 	if (unlikely(ret))
 		remove_waiter(lock, waiter);
 
-- 
2.1.4



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