trace: add a way to enable or disable the stack tracer

Impact: enhancement to stack tracer

The stack tracer currently is either on when configured in or
off when it is not. It can not be disabled when it is configured on.
(besides disabling the function tracer that it uses)

This patch adds a way to enable or disable the stack tracer at
run time. It defaults off on bootup, but a kernel parameter 'stacktrace'
has been added to enable it on bootup.

A new sysctl has been added "kernel.stack_tracer_enabled" to let
the user enable or disable the stack tracer at run time.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/kernel-parameters.txt

@@ -89,6 +89,7 @@ parameter is applicable:
 	SPARC	Sparc architecture is enabled.
 	SWSUSP	Software suspend (hibernation) is enabled.
 	SUSPEND	System suspend states are enabled.
+	FTRACE	Function tracing enabled.
 	TS	Appropriate touchscreen support is enabled.
 	USB	USB support is enabled.
 	USBHID	USB Human Interface Device support is enabled.
@@ -2173,6 +2174,9 @@ and is between 256 and 4096 characters. It is defined in the file
 	st=		[HW,SCSI] SCSI tape parameters (buffers, etc.)
 			See Documentation/scsi/st.txt.
 
+	stacktrace	[FTRACE]
+			Enabled the stack tracer on boot up.
+
 	sti=		[PARISC,HW]
 			Format: <num>
 			Set the STI (builtin display/keyboard on the HP-PARISC

include/linux/ftrace.h

@@ -86,6 +86,14 @@ static inline void ftrace_stop(void) { }
 static inline void ftrace_start(void) { }
 #endif /* CONFIG_FUNCTION_TRACER */
 
+#ifdef CONFIG_STACK_TRACER
+extern int stack_tracer_enabled;
+int
+stack_trace_sysctl(struct ctl_table *table, int write,
+		   struct file *file, void __user *buffer, size_t *lenp,
+		   loff_t *ppos);
+#endif
+
 #ifdef CONFIG_DYNAMIC_FTRACE
 /* asm/ftrace.h must be defined for archs supporting dynamic ftrace */
 #include <asm/ftrace.h>

kernel/sysctl.c

@@ -487,6 +487,16 @@ static struct ctl_table kern_table[] = {
 		.proc_handler	= &ftrace_enable_sysctl,
 	},
 #endif
+#ifdef CONFIG_STACK_TRACER
+	{
+		.ctl_name	= CTL_UNNUMBERED,
+		.procname	= "stack_tracer_enabled",
+		.data		= &stack_tracer_enabled,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &stack_trace_sysctl,
+	},
+#endif
 #ifdef CONFIG_TRACING
 	{
 		.ctl_name	= CTL_UNNUMBERED,

kernel/trace/Kconfig

@@ -244,10 +244,15 @@ config STACK_TRACER
 
 	  This tracer works by hooking into every function call that the
 	  kernel executes, and keeping a maximum stack depth value and
-	  stack-trace saved. Because this logic has to execute in every
-	  kernel function, all the time, this option can slow down the
-	  kernel measurably and is generally intended for kernel
-	  developers only.
+	  stack-trace saved.  If this is configured with DYNAMIC_FTRACE
+	  then it will not have any overhead while the stack tracer
+	  is disabled.
+
+	  To enable the stack tracer on bootup, pass in 'stacktrace'
+	  on the kernel command line.
+
+	  The stack tracer can also be enabled or disabled via the
+	  sysctl kernel.stack_tracer_enabled
 
 	  Say N if unsure.
 

kernel/trace/trace_stack.c

@@ -10,6 +10,7 @@
 #include <linux/debugfs.h>
 #include <linux/ftrace.h>
 #include <linux/module.h>
+#include <linux/sysctl.h>
 #include <linux/init.h>
 #include <linux/fs.h>
 #include "trace.h"
@@ -31,6 +32,10 @@ static raw_spinlock_t max_stack_lock =
 
 static int stack_trace_disabled __read_mostly;
 static DEFINE_PER_CPU(int, trace_active);
+static DEFINE_MUTEX(stack_sysctl_mutex);
+
+int stack_tracer_enabled;
+static int last_stack_tracer_enabled;
 
 static inline void check_stack(void)
 {
@@ -174,7 +179,7 @@ stack_max_size_write(struct file *filp, const char __user *ubuf,
 	return count;
 }
 
-static struct file_operations stack_max_size_fops = {
+static const struct file_operations stack_max_size_fops = {
 	.open		= tracing_open_generic,
 	.read		= stack_max_size_read,
 	.write		= stack_max_size_write,
@@ -272,7 +277,7 @@ static int t_show(struct seq_file *m, void *v)
 	return 0;
 }
 
-static struct seq_operations stack_trace_seq_ops = {
+static const struct seq_operations stack_trace_seq_ops = {
 	.start		= t_start,
 	.next		= t_next,
 	.stop		= t_stop,
@@ -288,12 +293,48 @@ static int stack_trace_open(struct inode *inode, struct file *file)
 	return ret;
 }
 
-static struct file_operations stack_trace_fops = {
+static const struct file_operations stack_trace_fops = {
 	.open		= stack_trace_open,
 	.read		= seq_read,
 	.llseek		= seq_lseek,
 };
 
+int
+stack_trace_sysctl(struct ctl_table *table, int write,
+		   struct file *file, void __user *buffer, size_t *lenp,
+		   loff_t *ppos)
+{
+	int ret;
+
+	mutex_lock(&stack_sysctl_mutex);
+
+	ret  = proc_dointvec(table, write, file, buffer, lenp, ppos);
+
+	if (ret || !write ||
+	    (last_stack_tracer_enabled == stack_tracer_enabled))
+		goto out;
+
+	last_stack_tracer_enabled = stack_tracer_enabled;
+
+	if (stack_tracer_enabled)
+		register_ftrace_function(&trace_ops);
+	else
+		unregister_ftrace_function(&trace_ops);
+
+ out:
+	mutex_unlock(&stack_sysctl_mutex);
+	return ret;
+}
+
+static int start_stack_trace __initdata;
+
+static __init int enable_stacktrace(char *str)
+{
+	start_stack_trace = 1;
+	return 1;
+}
+__setup("stacktrace", enable_stacktrace);
+
 static __init int stack_trace_init(void)
 {
 	struct dentry *d_tracer;
@@ -311,7 +352,10 @@ static __init int stack_trace_init(void)
 	if (!entry)
 		pr_warning("Could not create debugfs 'stack_trace' entry\n");
 
+	if (start_stack_trace) {
 		register_ftrace_function(&trace_ops);
+		stack_tracer_enabled = 1;
+	}
 
 	return 0;
 }