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

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Fri Feb 27, 2026 at 11:40 AM EST, Sun Jian wrote:
> 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.
>
> Tested:
>   ./test_progs -t bpf_cookie/perf_event -vv (30 runs): 0 failures
>
> Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

Nit below, but change makes sense.

> ---
>  .../selftests/bpf/prog_tests/bpf_cookie.c       | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c b/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c
> index 50f5e11e6e65..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;

Nit: AFAICT most of these changes are unnecessary, you could just bump
the cycle count to 100M and add the barrier. Moving everything to longs
and factoring out the loop count is unnecessary.

>  	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)
> @@ -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");
>