| From foo@baz Sat Nov 10 11:24:34 PST 2018 |
| From: Waiman Long <longman@redhat.com> |
| Date: Thu, 18 Oct 2018 21:45:17 -0400 |
| Subject: locking/lockdep: Fix debug_locks off performance problem |
| |
| 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 |
| @@ -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; |