selftests/bpf: Check for timeout in perf_link test

commit e6c209d upstream.

Recently perf_link test started unreliably failing on libbpf CI:
  * https://github.com/libbpf/libbpf/actions/runs/11260672407/job/31312405473
  * https://github.com/libbpf/libbpf/actions/runs/11260992334/job/31315514626
  * https://github.com/libbpf/libbpf/actions/runs/11263162459/job/31320458251

Part of the test is running a dummy loop for a while and then checking
for a counter incremented by the test program.

Instead of waiting for an arbitrary number of loop iterations once,
check for the test counter in a loop and use get_time_ns() helper to
enforce a 100ms timeout.

v1: https://lore.kernel.org/bpf/zuRd072x9tumn2iN4wDNs5av0nu5nekMNV4PkR-YwCT10eFFTrUtZBRkLWFbrcCe7guvLStGQlhibo8qWojCO7i2-NGajes5GYIyynexD-w=@pm.me/

Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241011153104.249800-1-ihor.solodrai@pm.me
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: theihor

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

@@ -4,8 +4,12 @@
 #include <pthread.h>
 #include <sched.h>
 #include <test_progs.h>
+#include "testing_helpers.h"
 #include "test_perf_link.skel.h"
 
+#define BURN_TIMEOUT_MS 100
+#define BURN_TIMEOUT_NS BURN_TIMEOUT_MS * 1000000
+
 static void burn_cpu(void)
 {
 	volatile int j = 0;
@@ -32,6 +36,7 @@ void serial_test_perf_link(void)
 	int run_cnt_before, run_cnt_after;
 	struct bpf_link_info info;
 	__u32 info_len = sizeof(info);
+	__u64 timeout_time_ns;
 
 	/* create perf event */
 	memset(&attr, 0, sizeof(attr));
@@ -63,8 +68,14 @@ void serial_test_perf_link(void)
 	ASSERT_GT(info.prog_id, 0, "link_prog_id");
 
 	/* ensure we get at least one perf_event prog execution */
-	burn_cpu();
-	ASSERT_GT(skel->bss->run_cnt, 0, "run_cnt");
+	timeout_time_ns = get_time_ns() + BURN_TIMEOUT_NS;
+	while (true) {
+		burn_cpu();
+		if (skel->bss->run_cnt > 0)
+			break;
+	        if (!ASSERT_LT(get_time_ns(), timeout_time_ns, "run_cnt_timeout"))
+			break;
+	}
 
 	/* perf_event is still active, but we close link and BPF program
 	 * shouldn't be executed anymore