function-graph: ignore times across schedule

Impact: more accurate timings

The current method of function graph tracing does not take into
account the time spent when a task is not running. This shows functions
that call schedule have increased costs:

 3) + 18.664 us   |      }
 ------------------------------------------
 3)    <idle>-0    =>  kblockd-123
 ------------------------------------------

 3)               |      finish_task_switch() {
 3)   1.441 us    |        _spin_unlock_irq();
 3)   3.966 us    |      }
 3) ! 2959.433 us |    }
 3) ! 2961.465 us |  }

This patch uses the tracepoint in the scheduling context switch to
account for time that has elapsed while a task is scheduled out.
Now we see:

 ------------------------------------------
 3)    <idle>-0    =>  edac-po-1067
 ------------------------------------------

 3)               |      finish_task_switch() {
 3)   0.685 us    |        _spin_unlock_irq();
 3)   2.331 us    |      }
 3) + 41.439 us   |    }
 3) + 42.663 us   |  }

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 89cd308cc7a5..471e36d30123 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1409,6 +1409,8 @@ struct task_struct {
         int curr_ret_stack;
         /* Stack of return addresses for return function tracing */
         struct ftrace_ret_stack *ret_stack;
+        /* time stamp for last schedule */
+        unsigned long long ftrace_timestamp;
         /*
          * Number of functions that haven't been traced
          * because of depth overrun.