From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org, Waiman Long <longman@redhat.com>
Subject: [PATCH 1/2] locking/lockdep: Fix debug_locks off performance problem
Date: Thu, 18 Oct 2018 21:45:17 -0400 [thread overview]
Message-ID: <1539913518-15598-1-git-send-email-longman@redhat.com> (raw)
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>
---
kernel/locking/lockdep.c | 4 ++--
lib/debug_locks.c | 2 +-
2 files changed, 3 insertions(+), 3 deletions(-)
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index be76f47..1efada2 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4066,7 +4066,7 @@ void lock_contended(struct lockdep_map *lock, unsigned long ip)
{
unsigned long flags;
- if (unlikely(!lock_stat))
+ if (unlikely(!lock_stat || !debug_locks))
return;
if (unlikely(current->lockdep_recursion))
@@ -4086,7 +4086,7 @@ void lock_acquired(struct lockdep_map *lock, unsigned long ip)
{
unsigned long flags;
- if (unlikely(!lock_stat))
+ if (unlikely(!lock_stat || !debug_locks))
return;
if (unlikely(current->lockdep_recursion))
diff --git a/lib/debug_locks.c b/lib/debug_locks.c
index 96c4c63..124fdf2 100644
--- a/lib/debug_locks.c
+++ b/lib/debug_locks.c
@@ -37,7 +37,7 @@
*/
int debug_locks_off(void)
{
- if (__debug_locks_off()) {
+ if (debug_locks && __debug_locks_off()) {
if (!debug_locks_silent) {
console_verbose();
return 1;
--
1.8.3.1
next reply other threads:[~2018-10-19 1:45 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-19 1:45 Waiman Long [this message]
2018-10-19 1:45 ` [PATCH 2/2] locking/lockdep: Make global debug_locks* variables read-mostly Waiman Long
2018-10-19 9:34 ` [tip:locking/core] " tip-bot for Waiman Long
2018-10-19 9:33 ` [tip:locking/core] locking/lockdep: Fix debug_locks off performance problem tip-bot for Waiman Long
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