kmemleak : Make kmemleak_stack_scan optional using config

Currently we have kmemleak_stack_scan enabled by default.
This can hog the cpu with pre-emption disabled for a long
time starving other tasks.

Make this optional at compile time, since if required
we can always write to sysfs entry and enable this option.

Change-Id: Ie30447861c942337c7ff25ac269b6025a527e8eb
Signed-off-by: Vignesh Radhakrishnan <vigneshr@codeaurora.org>
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>

lib/Kconfig.debug

@@ -934,6 +934,15 @@ config DEBUG_MODULE_SCAN_OFF
 	  run with irq/preemption disabled for considerable amount
 	  of time.
 
+config DEBUG_TASK_STACK_SCAN_OFF
+	bool "Disable kmemleak task stack scan by default"
+	depends on DEBUG_KMEMLEAK
+	help
+	  Say Y here to disable kmemleak task stack scan by default
+	  at compile time. It can be enabled later if required by
+	  writing to the debugfs entry :
+	  echo "stack=on" > /sys/kernel/debug/kmemleak.
+
 config DEBUG_PREEMPT
 	bool "Debug preemptible kernel"
 	depends on DEBUG_KERNEL && PREEMPT && TRACE_IRQFLAGS_SUPPORT

mm/kmemleak.c

@@ -215,8 +215,18 @@ static unsigned long jiffies_min_age;
 static unsigned long jiffies_last_scan;
 /* delay between automatic memory scannings */
 static signed long jiffies_scan_wait;
-/* enables or disables the task stacks scanning */
+
+/* Enables or disables the task stacks scanning.
+ * Set to 1 if at compile time we want it enabled.
+ * Else set to 0 to have it disabled by default.
+ * This can be enabled by writing to "stack=on" using
+ * kmemleak debugfs entry.*/
+#ifdef CONFIG_DEBUG_TASK_STACK_SCAN_OFF
+static int kmemleak_stack_scan;
+#else
 static int kmemleak_stack_scan = 1;
+#endif
+
 /* protects the memory scanning, parameters and debug/kmemleak file access */
 static DEFINE_MUTEX(scan_mutex);
 /* setting kmemleak=on, will set this var, skipping the disable */