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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Waiman Long <longman@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will.deacon@arm.com>,
        Ingo Molnar <mingo@kernel.org>, Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.19 050/361] locking/lockdep: Fix debug_locks off performance problem
Date:   Sun, 11 Nov 2018 14:16:37 -0800
Message-Id: <20181111221627.042283758@linuxfoundation.org>
In-Reply-To: <20181111221619.915519183@linuxfoundation.org>
References: <20181111221619.915519183@linuxfoundation.org>
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4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Waiman Long <longman@redhat.com>

[ Upstream commit 9506a7425b094d2f1d9c877ed5a78f416669269b ]

It was found that when debug_locks was turned off because of a problem
found by the lockdep code, the system performance could drop quite
significantly when the lock_stat code was also configured into the
kernel. For instance, parallel kernel build time on a 4-socket x86-64
server nearly doubled.

Further analysis into the cause of the slowdown traced back to the
frequent call to debug_locks_off() from the __lock_acquired() function
probably due to some inconsistent lockdep states with debug_locks
off. The debug_locks_off() function did an unconditional atomic xchg
to write a 0 value into debug_locks which had already been set to 0.
This led to severe cacheline contention in the cacheline that held
debug_locks.  As debug_locks is being referenced in quite a few different
places in the kernel, this greatly slow down the system performance.

To prevent that trashing of debug_locks cacheline, lock_acquired()
and lock_contended() now checks the state of debug_locks before
proceeding. The debug_locks_off() function is also modified to check
debug_locks before calling __debug_locks_off().

Signed-off-by: Waiman Long <longman@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/1539913518-15598-1-git-send-email-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/locking/lockdep.c |    4 ++--
 lib/debug_locks.c        |    2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4122,7 +4122,7 @@ void lock_contended(struct lockdep_map *
 {
 	unsigned long flags;
 
-	if (unlikely(!lock_stat))
+	if (unlikely(!lock_stat || !debug_locks))
 		return;
 
 	if (unlikely(current->lockdep_recursion))
@@ -4142,7 +4142,7 @@ void lock_acquired(struct lockdep_map *l
 {
 	unsigned long flags;
 
-	if (unlikely(!lock_stat))
+	if (unlikely(!lock_stat || !debug_locks))
 		return;
 
 	if (unlikely(current->lockdep_recursion))
--- a/lib/debug_locks.c
+++ b/lib/debug_locks.c
@@ -37,7 +37,7 @@ EXPORT_SYMBOL_GPL(debug_locks_silent);
  */
 int debug_locks_off(void)
 {
-	if (__debug_locks_off()) {
+	if (debug_locks && __debug_locks_off()) {
 		if (!debug_locks_silent) {
 			console_verbose();
 			return 1;