linux-kernel - [PATCH] rtmutex: rt_mutex_slowlock() is mostly called as TASK

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Date:   Thu, 19 Jan 2017 11:32:34 -0500
From:   "Steven Rostedt (VMware)" <rostedt@...dmis.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH] rtmutex: rt_mutex_slowlock() is mostly called as
 TASK_INTERRUPTIBLE

Running my likely/unlikely profiler for 3 weeks on two production
machines, I discovered that the unlikely() test in
__rt_mutex_slowlock() checking if state is TASK_INTERRUPTIBLE is hit
100% of the time, making it a very likely case.

The reason is, on a vanilla kernel, the majority case of calling
rt_mutex() is from the futex code. This code is always called as
TASK_INTERRUPTIBLE. In the -rt patch, this code is commonly called when
PREEMPT_RT is enabled with TASK_UNINTERRUPTIBLE. But that's not the
likely scenario.

The rt_mutex() code should be optimized for the common vanilla case,
and that is from a futex, with TASK_INTERRUPTIBLE as the state.

Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 1ec0f48..8de6288 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1115,7 +1115,7 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
 		 * TASK_INTERRUPTIBLE checks for signals and
 		 * timeout. Ignored otherwise.
 		 */
-		if (unlikely(state == TASK_INTERRUPTIBLE)) {
+		if (likely(state == TASK_INTERRUPTIBLE)) {
 			/* Signal pending? */
 			if (signal_pending(current))
 				ret = -EINTR;