4.9-stable review patch. If anyone has any objections, please let me know.
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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.co... 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 @@ -4010,7 +4010,7 @@ 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)) @@ -4030,7 +4030,7 @@ 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;