linux-kernel - [tip:locking/core] locking/rtmutex: Flip unlikely() branch to likely() in __rt_mutex

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Date:   Mon, 30 Jan 2017 03:53:18 -0800
From:   "tip-bot for Steven Rostedt (VMware)" <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     akpm@...ux-foundation.org, hpa@...or.com,
        linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
        mingo@...nel.org, tglx@...utronix.de, rostedt@...dmis.org,
        peterz@...radead.org
Subject: [tip:locking/core] locking/rtmutex: Flip unlikely() branch to
 likely() in __rt_mutex_slowlock()

Commit-ID:  4009f4b3a9d8b74547269f293e6a920adf278996
Gitweb:     http://git.kernel.org/tip/4009f4b3a9d8b74547269f293e6a920adf278996
Author:     Steven Rostedt (VMware) <rostedt@...dmis.org>
AuthorDate: Thu, 19 Jan 2017 11:32:34 -0500
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 30 Jan 2017 11:42:59 +0100

locking/rtmutex: Flip unlikely() branch to likely() in __rt_mutex_slowlock()

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/20170119113234.1efeedd1@gandalf.local.home
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/locking/rtmutex.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 2f443ed..d340be3 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1179,7 +1179,7 @@ __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;