selftests/bpf: Fix test_progs send_signal flakiness with nmi mode

Alexei observed that test_progs send_signal may fail if run
with command line "./test_progs" and the tests will pass
if just run "./test_progs -n 40".

I observed similar issue with nmi subtest failure
and added a delay 100 us in Commit ab8b7f0cb3
("tools/bpf: Add self tests for bpf_send_signal_thread()")
and the problem is gone for me. But the issue still exists
in Alexei's testing environment.

The current code uses sample_freq = 50 (50 events/second), which
may not be enough. But if the sample_freq value is larger than
sysctl kernel/perf_event_max_sample_rate, the perf_event_open
syscall will fail.

This patch changed nmi perf testing to use sample_period = 1,
which means trying to sampling every event. This seems fixing
the issue.

Fixes: ab8b7f0cb3 ("tools/bpf: Add self tests for bpf_send_signal_thread()")
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200116174004.1522812-1-yhs@fb.com

tools/testing/selftests/bpf/prog_tests/send_signal.c

@@ -76,9 +76,6 @@ static void test_send_signal_common(struct perf_event_attr *attr,
 	if (CHECK(!skel, "skel_open_and_load", "skeleton open_and_load failed\n"))
 		goto skel_open_load_failure;
 
-	/* add a delay for child thread to ramp up */
-	usleep(100);
-
 	if (!attr) {
 		err = test_send_signal_kern__attach(skel);
 		if (CHECK(err, "skel_attach", "skeleton attach failed\n")) {
@@ -155,8 +152,7 @@ static void test_send_signal_perf(bool signal_thread)
 static void test_send_signal_nmi(bool signal_thread)
 {
 	struct perf_event_attr attr = {
-		.sample_freq = 50,
-		.freq = 1,
+		.sample_period = 1,
 		.type = PERF_TYPE_HARDWARE,
 		.config = PERF_COUNT_HW_CPU_CYCLES,
 	};