tracing/function-return-tracer: store return stack into task_struct and allocate it dynamically

Impact: use deeper function tracing depth safely

Some tests showed that function return tracing needed a more deeper depth
of function calls. But it could be unsafe to store these return addresses
to the stack.

So these arrays will now be allocated dynamically into task_struct of current
only when the tracer is activated.

Typical scheme when tracer is activated:
- allocate a return stack for each task in global list.
- fork: allocate the return stack for the newly created task
- exit: free return stack of current
- idle init: same as fork

I chose a default depth of 50. I don't have overruns anymore.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 16 insertions, 12 deletions

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index f7ba4ea5e128..2ba259b2defa 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -323,6 +323,8 @@ struct ftrace_retfunc {
 };
 
 #ifdef CONFIG_FUNCTION_RET_TRACER
+#define FTRACE_RETFUNC_DEPTH 50
+#define FTRACE_RETSTACK_ALLOC_SIZE 32
 /* Type of a callback handler of tracing return function */
 typedef void (*trace_function_return_t)(struct ftrace_retfunc *);
 
@@ -330,6 +332,9 @@ extern int register_ftrace_return(trace_function_return_t func);
 /* The current handler in use */
 extern trace_function_return_t ftrace_function_return;
 extern void unregister_ftrace_return(void);
+
+extern void ftrace_retfunc_init_task(struct task_struct *t);
+extern void ftrace_retfunc_exit_task(struct task_struct *t);
 #endif
 
 #endif /* _LINUX_FTRACE_H */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index c8e0db464206..bee1e93c95ad 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1352,6 +1352,17 @@ struct task_struct {
         unsigned long default_timer_slack_ns;
 
         struct list_head        *scm_work_list;
+#ifdef CONFIG_FUNCTION_RET_TRACER
+        /* Index of current stored adress in ret_stack */
+        int curr_ret_stack;
+        /* Stack of return addresses for return function tracing */
+        struct ftrace_ret_stack *ret_stack;
+        /*
+         * Number of functions that haven't been traced
+         * because of depth overrun.
+         */
+        atomic_t trace_overrun;
+#endif
 };
 
 /*
@@ -2006,18 +2017,6 @@ static inline void setup_thread_stack(struct task_struct *p, struct task_struct
 {
         *task_thread_info(p) = *task_thread_info(org);
         task_thread_info(p)->task = p;
-
-#ifdef CONFIG_FUNCTION_RET_TRACER
-        /*
-         * When fork() creates a child process, this function is called.
-         * But the child task may not inherit the return adresses traced
-         * by the return function tracer because it will directly execute
-         * in userspace and will not return to kernel functions its parent
-         * used.
-         */
-        task_thread_info(p)->curr_ret_stack = -1;
-        atomic_set(&task_thread_info(p)->trace_overrun, 0);
-#endif
 }
 
 static inline unsigned long *end_of_stack(struct task_struct *p)