[PATCH v2 2/2] selftests/bpf: bpf_cookie: make perf_event subtest trigger reliably

[Sun Jian <sun.jian.kdev@gmail.com>]

The perf_event subtest relies on SW_CPU_CLOCK sampling to trigger the BPF
program, but the current CPU burn loop can be too short on slower systems
and may fail to generate any overflow sample. This leaves pe_res unchanged
and makes the test flaky.

Make burn_cpu() take a loop count and use a longer burn only for the
perf_event subtest. Also scope perf_event_open() to the current task to
avoid wasting samples on unrelated activity.

Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>

---
Changes in v2:

Move the perf_event_open() argument change here from patch 1/2.

v1: <https://lore.kernel.org/lkml/20260227164037.84110-1-sun.jian.kdev@gmail.com/>
---
 .../selftests/bpf/prog_tests/bpf_cookie.c     | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c b/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c
index b7643a5bf7ad..35adc3f6d443 100644
--- a/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c
+++ b/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c
@@ -6,6 +6,7 @@
 #include <sys/syscall.h>
 #include <sys/mman.h>
 #include <unistd.h>
+#include <linux/compiler.h>
 #include <test_progs.h>
 #include <network_helpers.h>
 #include <bpf/btf.h>
@@ -431,11 +432,12 @@ static void tp_subtest(struct test_bpf_cookie *skel)
 	bpf_link__destroy(link3);
 }
 
-static void burn_cpu(void)
+static void burn_cpu(long loops)
 {
-	volatile int j = 0;
+	long j = 0;
 	cpu_set_t cpu_set;
-	int i, err;
+	long i;
+	int err;
 
 	/* generate some branches on cpu 0 */
 	CPU_ZERO(&cpu_set);
@@ -443,9 +445,10 @@ static void burn_cpu(void)
 	err = pthread_setaffinity_np(pthread_self(), sizeof(cpu_set), &cpu_set);
 	ASSERT_OK(err, "set_thread_affinity");
 
-	/* spin the loop for a while (random high number) */
-	for (i = 0; i < 1000000; ++i)
+	for (i = 0; i < loops; ++i) {
 		++j;
+		barrier();
+	}
 }
 
 static void pe_subtest(struct test_bpf_cookie *skel)
@@ -461,7 +464,7 @@ static void pe_subtest(struct test_bpf_cookie *skel)
 	attr.type = PERF_TYPE_SOFTWARE;
 	attr.config = PERF_COUNT_SW_CPU_CLOCK;
 	attr.sample_period = 100000;
-	pfd = syscall(__NR_perf_event_open, &attr, -1, 0, -1, PERF_FLAG_FD_CLOEXEC);
+	pfd = syscall(__NR_perf_event_open, &attr, 0, -1, -1, PERF_FLAG_FD_CLOEXEC);
 	if (!ASSERT_GE(pfd, 0, "perf_fd"))
 		goto cleanup;
 
@@ -470,7 +473,7 @@ static void pe_subtest(struct test_bpf_cookie *skel)
 	if (!ASSERT_OK_PTR(link, "link1"))
 		goto cleanup;
 
-	burn_cpu(); /* trigger BPF prog */
+	burn_cpu(100000000L); /* trigger BPF prog */
 
 	ASSERT_EQ(skel->bss->pe_res, 0x100000, "pe_res1");
 
@@ -489,7 +492,7 @@ static void pe_subtest(struct test_bpf_cookie *skel)
 	if (!ASSERT_OK_PTR(link, "link2"))
 		goto cleanup;
 
-	burn_cpu(); /* trigger BPF prog */
+	burn_cpu(100000000L); /* trigger BPF prog */
 
 	ASSERT_EQ(skel->bss->pe_res, 0x200000, "pe_res2");
 
-- 
2.43.0