selftests/bpf: Reduce flakyness in timer_mim

This patch extends wait time in timer_mim. As observed in slow CI environment,
it is possible to have interrupt/preemption long enough to cause the test to
fail, almost 1 failure in 5 runs.
Signed-off-by: Yucong Sun <fallentree@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210823213629.3519641-1-fallentree@fb.com

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

@@ -23,8 +23,12 @@ static int timer_mim(struct timer_mim *timer_skel)
 
 	/* check that timer_cb[12] are incrementing 'cnt' */
 	cnt1 = READ_ONCE(timer_skel->bss->cnt);
-	usleep(200); /* 100 times more than interval */
-	cnt2 = READ_ONCE(timer_skel->bss->cnt);
+	for (int i = 0; i < 100; i++) {
+		cnt2 = READ_ONCE(timer_skel->bss->cnt);
+		if (cnt2 != cnt1)
+			break;
+		usleep(200); /* 100 times more than interval */
+	}
 	ASSERT_GT(cnt2, cnt1, "cnt");
 
 	ASSERT_EQ(timer_skel->bss->err, 0, "err");
@@ -37,8 +41,12 @@ static int timer_mim(struct timer_mim *timer_skel)
 
 	/* check that timer_cb[12] are no longer running */
 	cnt1 = READ_ONCE(timer_skel->bss->cnt);
-	usleep(200);
-	cnt2 = READ_ONCE(timer_skel->bss->cnt);
+	for (int i = 0; i < 100; i++) {
+		usleep(200); /* 100 times more than interval */
+		cnt2 = READ_ONCE(timer_skel->bss->cnt);
+		if (cnt2 == cnt1)
+			break;
+	}
 	ASSERT_EQ(cnt2, cnt1, "cnt");
 
 	return 0;