[PATCH] Kconfig.debug: Set DEBUG_MUTEX to off by default

DEBUG_MUTEX flag is on by default in current kernel configuration.

During performance testing, we saw mutex debug functions like
mutex_debug_check_no_locks_freed (called by kfree()) is expensive as it
goes through a global list of memory areas with mutex lock and do the
checking.  For benchmarks such as Volanomark and Hackbench, we have seen
more than 40% drop in performance on some platforms.  We suggest to set
DEBUG_MUTEX off by default.  Or at least do that later when we feel that
the mutex changes in the current code have stabilized.
Signed-off-by: NTim Chen <tim.c.chen@intel.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

lib/Kconfig.debug

@@ -101,7 +101,7 @@ config DEBUG_PREEMPT
 
 config DEBUG_MUTEXES
 	bool "Mutex debugging, deadlock detection"
-	default y
+	default n
 	depends on DEBUG_KERNEL
 	help
 	 This allows mutex semantics violations and mutex related deadlocks