perf test: Handle fd gaps in test__dso_data_reopen

https://github.com/beaker-project/restraint/issues/215 describes a file
descriptor leak which revealed the test failure described here.

The 'DSO data reopen' perf test assumes that RLIMIT_NOFILE limits the
number of open file descriptors, but it actually limits newly opened
file descriptors. When the file descriptor limit is reduced, file
descriptors already open remain open regardless of the new limit. This
test failure does not occur if open file descriptors are contiguous,
beginning at zero.

The following command triggers this perf test failure.

perf test 'DSO data reopen' 3>/dev/null 8>/dev/null

This patch determines the file descriptor limit by opening four files
and then closing them. The limit is set to the fourth file descriptor,
leaving only the first three available because any newly opened file
descriptor must be less than the limit.
Signed-off-by: Eirik Fuller <efuller@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Michael Petlan <mpetlan@redhat.com>
LPU-Reference: 20210626023825.1398547-1-efuller@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/dso-data.c

@@ -308,10 +308,20 @@ int test__dso_data_cache(struct test *test __maybe_unused, int subtest __maybe_u
 	return 0;
 }
 
+static long new_limit(int count)
+{
+	int fd = open("/dev/null", O_RDONLY);
+	long ret = fd;
+	if (count > 0)
+		ret = new_limit(--count);
+	close(fd);
+	return ret;
+}
+
 int test__dso_data_reopen(struct test *test __maybe_unused, int subtest __maybe_unused)
 {
 	struct machine machine;
-	long nr_end, nr = open_files_cnt();
+	long nr_end, nr = open_files_cnt(), lim = new_limit(3);
 	int fd, fd_extra;
 
 #define dso_0 (dsos[0])
@@ -334,7 +344,7 @@ int test__dso_data_reopen(struct test *test __maybe_unused, int subtest __maybe_
 
 	/* Make sure we are able to open 3 fds anyway */
 	TEST_ASSERT_VAL("failed to set file limit",
-			!set_fd_limit((nr + 3)));
+			!set_fd_limit((lim)));
 
 	TEST_ASSERT_VAL("failed to create dsos\n", !dsos__create(3, TEST_FILE_SIZE));