[PATCH v2] Poll for monitored tasks being alive in fork mode

[Jin Yao <yao.jin@linux.intel.com>]

Following test shows the stat keeps running even if no longer
task to monitor (mgen exits at ~5s).

perf stat -e cycles -p `pgrep mgen` -I1000 -- sleep 10
            time             counts unit events
     1.000148916      1,308,365,864      cycles
     2.000379171      1,297,269,875      cycles
     3.000556719      1,297,187,078      cycles
     4.000914241        761,261,827      cycles
     5.001306091      <not counted>      cycles
     6.001676881      <not counted>      cycles
     7.002046336      <not counted>      cycles
     8.002405651      <not counted>      cycles
     9.002766625      <not counted>      cycles
    10.001395827      <not counted>      cycles

We'd better finish stat immediately if there's no longer task to
monitor.

After:

perf stat -e cycles -p `pgrep mgen` -I1000 -- sleep 10
            time             counts unit events
     1.000180062      1,236,592,661      cycles
     2.000421539      1,223,733,572      cycles
     3.000609910      1,297,047,663      cycles
     4.000807545      1,297,215,816      cycles
     5.001001578      1,297,208,032      cycles
     6.001390345        582,343,659      cycles
sleep: Terminated

Now the stat exits immediately when the monitored tasks ends.

v2:
---
There is a potential race condition window, that is the
child_pid set with -1 and kill(-1, SIGTERM) may happen.
Jiri provides a simple method which stores the child_pid to
a local variable and check the local variable before kill().
At least, we can avoid the kill(-1, SIGTERM) case.

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
---
 tools/perf/builtin-stat.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index 63a3afc..bd5fb8e 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -553,6 +553,15 @@ static int __run_perf_stat(int argc, const char **argv, int run_idx)
 
 		if (interval || timeout) {
 			while (!waitpid(child_pid, &status, WNOHANG)) {
+				if (!is_target_alive(&target,
+					evsel_list->threads)) {
+					int pid = child_pid;
+
+					if (pid != -1)
+						kill(pid, SIGTERM);
+					break;
+				}
+
 				nanosleep(&ts, NULL);
 				if (timeout)
 					break;
-- 
2.7.4