tracing: Add trace_dump_stack()

I've been asked a few times about how to find out what is calling
some location in the kernel. One way is to use dynamic function tracing
and implement the func_stack_trace. But this only finds out who is
calling a particular function. It does not tell you who is calling
that function and entering a specific if conditional.

I have myself implemented a quick version of trace_dump_stack() for
this purpose a few times, and just needed it now. This is when I realized
that this would be a good tool to have in the kernel like trace_printk().

Using trace_dump_stack() is similar to dump_stack() except that it
writes to the trace buffer instead and can be used in critical locations.

For example:

@@ -5485,8 +5485,12 @@ need_resched_nonpreemptible:
 	if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
 		if (unlikely(signal_pending_state(prev->state, prev)))
 			prev->state = TASK_RUNNING;
-		else
+		else {
 			deactivate_task(rq, prev, 1);
+			trace_printk("Deactivating task %s:%d\n",
+				     prev->comm, prev->pid);
+			trace_dump_stack();
+		}
 		switch_count = &prev->nvcsw;
 	}

Produces:

           <...>-3249  [001]   296.105269: schedule: Deactivating task ntpd:3249
           <...>-3249  [001]   296.105270: <stack trace>
 => schedule
 => schedule_hrtimeout_range
 => poll_schedule_timeout
 => do_select
 => core_sys_select
 => sys_select
 => system_call_fastpath
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

include/linux/kernel.h

@@ -492,6 +492,8 @@ extern int
 __trace_printk(unsigned long ip, const char *fmt, ...)
 	__attribute__ ((format (printf, 2, 3)));
 
+extern void trace_dump_stack(void);
+
 /*
  * The double __builtin_constant_p is because gcc will give us an error
  * if we try to allocate the static variable to fmt if it is not a

kernel/trace/trace.c

@@ -1151,6 +1151,22 @@ void __trace_stack(struct trace_array *tr, unsigned long flags, int skip,
 	__ftrace_trace_stack(tr->buffer, flags, skip, pc);
 }
 
+/**
+ * trace_dump_stack - record a stack back trace in the trace buffer
+ */
+void trace_dump_stack(void)
+{
+	unsigned long flags;
+
+	if (tracing_disabled || tracing_selftest_running)
+		return 0;
+
+	local_save_flags(flags);
+
+	/* skipping 3 traces, seems to get us at the caller of this function */
+	__ftrace_trace_stack(global_trace.buffer, flags, 3, preempt_count());
+}
+
 void
 ftrace_trace_userstack(struct ring_buffer *buffer, unsigned long flags, int pc)
 {