From 9506a7425b094d2f1d9c877ed5a78f416669269b Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Thu, 18 Oct 2018 21:45:17 -0400 Subject: locking/lockdep: Fix debug_locks off performance problem 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 Cc: Andrew Morton Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Will Deacon Link: http://lkml.kernel.org/r/1539913518-15598-1-git-send-email-longman@redhat.com Signed-off-by: Ingo Molnar --- lib/debug_locks.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'lib') 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; -- cgit v1.2.3 From 01a14bda11add9dcd4a59200f13834d634559935 Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Thu, 18 Oct 2018 21:45:18 -0400 Subject: locking/lockdep: Make global debug_locks* variables read-mostly Make the frequently used lockdep global variable debug_locks read-mostly. As debug_locks_silent is sometime used together with debug_locks, it is also made read-mostly so that they can be close together. With false cacheline sharing, cacheline contention problem can happen depending on what get put into the same cacheline as debug_locks. Signed-off-by: Waiman Long Cc: Andrew Morton Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Will Deacon Link: http://lkml.kernel.org/r/1539913518-15598-2-git-send-email-longman@redhat.com Signed-off-by: Ingo Molnar --- include/linux/debug_locks.h | 4 ++-- lib/debug_locks.c | 4 ++-- 2 files changed, 4 insertions(+), 4 deletions(-) (limited to 'lib') diff --git a/include/linux/debug_locks.h b/include/linux/debug_locks.h index 120225e9a366..257ab3c92cb8 100644 --- a/include/linux/debug_locks.h +++ b/include/linux/debug_locks.h @@ -8,8 +8,8 @@ struct task_struct; -extern int debug_locks; -extern int debug_locks_silent; +extern int debug_locks __read_mostly; +extern int debug_locks_silent __read_mostly; static inline int __debug_locks_off(void) diff --git a/lib/debug_locks.c b/lib/debug_locks.c index 124fdf238b3d..ce51749cc145 100644 --- a/lib/debug_locks.c +++ b/lib/debug_locks.c @@ -21,7 +21,7 @@ * that would just muddy the log. So we report the first one and * shut up after that. */ -int debug_locks = 1; +int debug_locks __read_mostly = 1; EXPORT_SYMBOL_GPL(debug_locks); /* @@ -29,7 +29,7 @@ EXPORT_SYMBOL_GPL(debug_locks); * 'silent failure': nothing is printed to the console when * a locking bug is detected. */ -int debug_locks_silent; +int debug_locks_silent __read_mostly; EXPORT_SYMBOL_GPL(debug_locks_silent); /* -- cgit v1.2.3