Commit 03889384 authored Dec 11, 2009 by Steven Rostedt Committed by Steven Rostedt Dec 11, 2009

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>

parent dd7f5943

include/linux/kernel.h

+2 −0

Original line number	Diff line number	Diff line
		@@ -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

+16 −0

Original line number	Diff line number	Diff line
		@@ -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)
		{

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