The function graph tracer is currently the most invasive tracer
in the ftrace family. It can easily overflow the buffer even with
10megs per CPU. This means that events can often be lost.

On start up, or after events are lost, if the function return is
recorded but the function enter was lost, all we get to see is the
exiting '}'.

Here is how a typical trace output starts:

 [tracing] cat trace
 # tracer: function_graph
 #
 # CPU  DURATION                  FUNCTION CALLS
 # |     |   |                     |   |   |   |
  0) + 91.897 us   |                  }
  0) ! 567.961 us  |                }
  0)   <========== |
  0) ! 579.083 us  |                _raw_spin_lock_irqsave();
  0)   4.694 us    |                _raw_spin_unlock_irqrestore();
  0) ! 594.862 us  |              }
  0) ! 603.361 us  |            }
  0) ! 613.574 us  |          }
  0) ! 623.554 us  |        }
  0)   3.653 us    |        fget_light();
  0)               |        sock_poll() {

There are a series of '}' with no matching "func() {". There's no information
to what functions these ending brackets belong to.

This patch adds a stack on the per cpu structure used in outputting
the function graph tracer to keep track of what function was outputted.
Then on a function exit event, it checks the depth to see if the
function exit has a matching entry event. If it does, then it only
prints the '}', otherwise it adds the function name after the '}'.

This allows function exit events to show what function they belong to
at trace output startup, when the entry was lost due to ring buffer
overflow, or even after a new task is scheduled in.

Here is what the above trace will look like after this patch:

 [tracing] cat trace
 # tracer: function_graph
 #
 # CPU  DURATION                  FUNCTION CALLS
 # |     |   |                     |   |   |   |
  0) + 91.897 us   |                  } (irq_exit)
  0) ! 567.961 us  |                } (smp_apic_timer_interrupt)
  0)   <========== |
  0) ! 579.083 us  |                _raw_spin_lock_irqsave();
  0)   4.694 us    |                _raw_spin_unlock_irqrestore();
  0) ! 594.862 us  |              } (add_wait_queue)
  0) ! 603.361 us  |            } (__pollwait)
  0) ! 613.574 us  |          } (tcp_poll)
  0) ! 623.554 us  |        } (sock_poll)
  0)   3.653 us    |        fget_light();
  0)               |        sock_poll() {

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>