Merge pull request iovisor#1687 from iovisor/yhs_dev2

fix test_libbcc failure with 4.15.

tests/cc/test_perf_event.cc

@@ -131,7 +131,11 @@ TEST_CASE("test attach perf event", "[bpf_perf_event]") {
  auto ret = bpf.get_hash_table<int, int>("ret");
  REQUIRE(ret[0] == 0);
  REQUIRE(counter.counter >= 0);
- REQUIRE(counter.enabled >= 1000000000);
+ // the program slept one second between perf_event attachment and detachment
+ // in the above, so the enabled counter should be 1000000000ns or
+ // more. But in reality, most of counters (if not all) are 9xxxxxxxx,
+ // and I also saw one 8xxxxxxxx. So let us a little bit conservative here.
+ REQUIRE(counter.enabled >= 800000000);
  REQUIRE(counter.running >= 0);
  REQUIRE(counter.running <= counter.enabled);
 #endif