| From | Greg Kroah-Hartman <> | Subject | [PATCH 4.19 050/361] locking/lockdep: Fix debug_locks off performance problem | Date | Sun, 11 Nov 2018 14:16:37 -0800 |
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4.19-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.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;
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