When function_graph is used for latency tracers, relative timestamp
is more straightforward than absolute timestamp as function trace
does. This change adds relative timestamp support to function_graph
and applies to latency tracers (wakeup and irqsoff).

Instead of:

 # tracer: irqsoff
 #
 # irqsoff latency trace v1.1.5 on 5.0.0-rc1-test
 # --------------------------------------------------------------------
 # latency: 521 us, #1125/1125, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:8)
 #    -----------------
 #    | task: swapper/2-0 (uid:0 nice:0 policy:0 rt_prio:0)
 #    -----------------
 #  => started at: __schedule
 #  => ended at:   _raw_spin_unlock_irq
 #
 #
 #                                       _-----=> irqs-off
 #                                      / _----=> need-resched
 #                                     | / _---=> hardirq/softirq
 #                                     || / _--=> preempt-depth
 #                                     ||| /
 #     TIME        CPU  TASK/PID       ||||  DURATION                  FUNCTION CALLS
 #      |          |     |    |        ||||   |   |                     |   |   |   |
   124.974306 |   2)  systemd-693   |  d..1  0.000 us    |  __schedule();
   124.974307 |   2)  systemd-693   |  d..1              |    rcu_note_context_switch() {
   124.974308 |   2)  systemd-693   |  d..1  0.487 us    |      rcu_preempt_deferred_qs();
   124.974309 |   2)  systemd-693   |  d..1  0.451 us    |      rcu_qs();
   124.974310 |   2)  systemd-693   |  d..1  2.301 us    |    }
[..]
   124.974826 |   2)    <idle>-0    |  d..2              |  finish_task_switch() {
   124.974826 |   2)    <idle>-0    |  d..2              |    _raw_spin_unlock_irq() {
   124.974827 |   2)    <idle>-0    |  d..2  0.000 us    |  _raw_spin_unlock_irq();
   124.974828 |   2)    <idle>-0    |  d..2  0.000 us    |  tracer_hardirqs_on();
   <idle>-0       2d..2  552us : <stack trace>
  => __schedule
  => schedule_idle
  => do_idle
  => cpu_startup_entry
  => start_secondary
  => secondary_startup_64

Show:

 # tracer: irqsoff
 #
 # irqsoff latency trace v1.1.5 on 5.0.0-rc1-test+
 # --------------------------------------------------------------------
 # latency: 511 us, #1053/1053, CPU#7 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:8)
 #    -----------------
 #    | task: swapper/7-0 (uid:0 nice:0 policy:0 rt_prio:0)
 #    -----------------
 #  => started at: __schedule
 #  => ended at:   _raw_spin_unlock_irq
 #
 #
 #                                       _-----=> irqs-off
 #                                      / _----=> need-resched
 #                                     | / _---=> hardirq/softirq
 #                                     || / _--=> preempt-depth
 #                                     ||| /
 #   REL TIME      CPU  TASK/PID       ||||  DURATION                  FUNCTION CALLS
 #      |          |     |    |        ||||   |   |                     |   |   |   |
         0 us |   7)   sshd-1704    |  d..1  0.000 us    |  __schedule();
         1 us |   7)   sshd-1704    |  d..1              |    rcu_note_context_switch() {
         1 us |   7)   sshd-1704    |  d..1  0.611 us    |      rcu_preempt_deferred_qs();
         2 us |   7)   sshd-1704    |  d..1  0.484 us    |      rcu_qs();
         3 us |   7)   sshd-1704    |  d..1  2.599 us    |    }
[..]
       509 us |   7)    <idle>-0    |  d..2              |  finish_task_switch() {
       510 us |   7)    <idle>-0    |  d..2              |    _raw_spin_unlock_irq() {
       510 us |   7)    <idle>-0    |  d..2  0.000 us    |  _raw_spin_unlock_irq();
       512 us |   7)    <idle>-0    |  d..2  0.000 us    |  tracer_hardirqs_on();
   <idle>-0       7d..2  543us : <stack trace>
  => __schedule
  => schedule_idle
  => do_idle
  => cpu_startup_entry
  => start_secondary
  => secondary_startup_64

Link: http://lkml.kernel.org/r/20190101154614.8887-2-changbin.du@gmail.comSigned-off-by: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>