From: Waiman Long <long...@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 <long...@redhat.com> Cc: Andrew Morton <a...@linux-foundation.org> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Will Deacon <will.dea...@arm.com> Link: http://lkml.kernel.org/r/1539913518-15598-1-git-send-email-long...@redhat.com Signed-off-by: Ingo Molnar <mi...@kernel.org> Signed-off-by: Sasha Levin <sas...@kernel.org> --- 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 6e171b547a80..774ab79d3ec7 100644 --- a/kernel/locking/lockdep.c +++ b/kernel/locking/lockdep.c @@ -3826,7 +3826,7 @@ 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)) @@ -3846,7 +3846,7 @@ 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 96c4c633d95e..124fdf238b3d 100644 --- 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; -- 2.17.1
