perf sched timehist: Fix invalid period calculation

When --time option is given with a value outside recorded time, the last
sample time (tprev) was set to that value and run time calculation might
be incorrect.  This is a problem of the first samples for each cpus
since it would skip the runtime update when tprev is 0.  But with --time
option it had non-zero (which is invalid) value so the calculation is
also incorrect.

For example, let's see the followging:

  $ perf sched timehist
             time    cpu  task name                       wait time  sch delay   run time
                          [tid/pid]                          (msec)     (msec)     (msec)
  --------------- ------  ------------------------------  ---------  ---------  ---------
      3195.968367 [0003]  <idle>                              0.000      0.000      0.000
      3195.968386 [0002]  Timer[4306/4277]                    0.000      0.000      0.018
      3195.968397 [0002]  Web Content[4277]                   0.000      0.000      0.000
      3195.968595 [0001]  JS Helper[4302/4277]                0.000      0.000      0.000
      3195.969217 [0000]  <idle>                              0.000      0.000      0.621
      3195.969251 [0001]  kworker/1:1H[291]                   0.000      0.000      0.033

The sample starts at 3195.968367 but when I gave a time interval from
3194 to 3196 (in sec) it will calculate the whole 2 second as runtime.
In below, 2 cpus accounted it as runtime, other 2 cpus accounted it as
idle time.

Before:

  $ perf sched timehist --time 3194,3196 -s | tail
  Idle stats:
      CPU  0 idle for   1995.991  msec
      CPU  1 idle for     20.793  msec
      CPU  2 idle for     30.191  msec
      CPU  3 idle for   1999.852  msec

      Total number of unique tasks: 23
  Total number of context switches: 128
             Total run time (msec): 3724.940

After:

  $ perf sched timehist --time 3194,3196 -s | tail
  Idle stats:
      CPU  0 idle for     10.811  msec
      CPU  1 idle for     20.793  msec
      CPU  2 idle for     30.191  msec
      CPU  3 idle for     18.337  msec

      Total number of unique tasks: 23
  Total number of context switches: 128
             Total run time (msec): 18.139

Committer notes:

Further testing:

Before:

  Idle stats:
      CPU  0 idle for    229.785  msec
      CPU  1 idle for    937.944  msec
      CPU  2 idle for    188.931  msec
      CPU  3 idle for    986.185  msec

  After:

  # perf sched timehist --time 40602,40603 -s | tail

  Idle stats:
      CPU  0 idle for    229.785  msec
      CPU  1 idle for    175.407  msec
      CPU  2 idle for    188.931  msec
      CPU  3 idle for    223.657  msec

      Total number of unique tasks: 68
  Total number of context switches: 814
             Total run time (msec): 97.688

  # for cpu in `seq 0 3` ; do echo -n "CPU $cpu idle for " ; perf sched timehist --time 40602,40603 | grep "\[000${cpu}\].*\<idle\>" | tr -s ' ' | cut -d' ' -f7 | awk '{entries++ ; s+=$1} END {print s " msec (entries: " entries ")"}' ; done
  CPU 0 idle for 229.721 msec (entries: 123)
  CPU 1 idle for 175.381 msec (entries: 65)
  CPU 2 idle for 188.903 msec (entries: 56)
  CPU 3 idle for 223.61 msec (entries: 102)

Difference due to the idle stats being accounted at nanoseconds precision while
the <idle> entries in 'perf sched timehist' are trucated at msec.usec.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Fixes: 853b740711 ("perf sched timehist: Add option to specify time window of interest")
Link: http://lkml.kernel.org/r/20161222060350.17655-2-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-sched.c

@@ -2405,7 +2405,7 @@ static int timehist_sched_change_event(struct perf_tool *tool,
 	if (ptime->start && ptime->start > t)
 		goto out;
 
-	if (ptime->start > tprev)
+	if (tprev && ptime->start > tprev)
 		tprev = ptime->start;
 
 	/*