Re: [PATCH bpf-next] selftests/bpf: improve reliability of test_perf_branches_no_hw()

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, Nov 19, 2025 at 02:35:40PM +0000, Matt Bobrowski wrote:
> Currently, test_perf_branches_no_hw() relies on the busy loop within
> test_perf_branches_common() being slow enough to allow at least one
> perf event sample tick to occur before starting to tear down the
> backing perf event BPF program. With a relatively small fixed
> iteration count of 1,000,000, this is not guaranteed on modern fast
> CPUs, resulting in the test run to subsequently fail with the
> following:
> 
> bpf_testmod.ko is already unloaded.
> Loading bpf_testmod.ko...
> Successfully loaded bpf_testmod.ko.
> test_perf_branches_common:PASS:test_perf_branches_load 0 nsec
> test_perf_branches_common:PASS:attach_perf_event 0 nsec
> test_perf_branches_common:PASS:set_affinity 0 nsec
> check_good_sample:PASS:output not valid 0 nsec
> check_good_sample:PASS:read_branches_size 0 nsec
> check_good_sample:PASS:read_branches_stack 0 nsec
> check_good_sample:PASS:read_branches_stack 0 nsec
> check_good_sample:PASS:read_branches_global 0 nsec
> check_good_sample:PASS:read_branches_global 0 nsec
> check_good_sample:PASS:read_branches_size 0 nsec
> test_perf_branches_no_hw:PASS:perf_event_open 0 nsec
> test_perf_branches_common:PASS:test_perf_branches_load 0 nsec
> test_perf_branches_common:PASS:attach_perf_event 0 nsec
> test_perf_branches_common:PASS:set_affinity 0 nsec
> check_bad_sample:FAIL:output not valid no valid sample from prog
> Summary: 0/1 PASSED, 0 SKIPPED, 1 FAILED
> Successfully unloaded bpf_testmod.ko.
> 
> On a modern CPU (i.e. one with a 3.5 GHz clock rate), executing 1
> million increments of a volatile integer can take significantly less
> than 1 millisecond. If the spin loop and detachment of the perf event
> BPF program elapses before the first 1 ms sampling interval elapses,
> the perf event will never end up firing. Fix this by bumping the loop
> iteration counter a little within test_perf_branches_common(), along
> with ensuring adding another loop termination condition which is
> directly influenced by the backing perf event BPF program
> executing. Notably, a concious decision was made to not adjust the
> sample_freq value as that is just not a reliable way to go about
> fixing the problem. It effectively still leaves the race window open.
> 
> Fixes: 67306f84ca78c ("selftests/bpf: Add bpf_read_branch_records() selftest")
> Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>

lgtm

Reviewed-by: Jiri Olsa <jolsa@kernel.org>

jirka


> ---
>  .../selftests/bpf/prog_tests/perf_branches.c     | 16 ++++++++++++++--
>  .../selftests/bpf/progs/test_perf_branches.c     |  3 +++
>  2 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/perf_branches.c b/tools/testing/selftests/bpf/prog_tests/perf_branches.c
> index bc24f83339d6..1d51ec5f171a 100644
> --- a/tools/testing/selftests/bpf/prog_tests/perf_branches.c
> +++ b/tools/testing/selftests/bpf/prog_tests/perf_branches.c
> @@ -15,6 +15,10 @@ static void check_good_sample(struct test_perf_branches *skel)
>  	int pbe_size = sizeof(struct perf_branch_entry);
>  	int duration = 0;
>  
> +	if (CHECK(!skel->bss->run_cnt, "invalid run_cnt",
> +		  "checked sample validity before prog run"))
> +		return;
> +
>  	if (CHECK(!skel->bss->valid, "output not valid",
>  		 "no valid sample from prog"))
>  		return;
> @@ -45,6 +49,10 @@ static void check_bad_sample(struct test_perf_branches *skel)
>  	int written_stack = skel->bss->written_stack_out;
>  	int duration = 0;
>  
> +	if (CHECK(!skel->bss->run_cnt, "invalid run_cnt",
> +		  "checked sample validity before prog run"))
> +		return;
> +
>  	if (CHECK(!skel->bss->valid, "output not valid",
>  		 "no valid sample from prog"))
>  		return;
> @@ -83,8 +91,12 @@ static void test_perf_branches_common(int perf_fd,
>  	err = pthread_setaffinity_np(pthread_self(), sizeof(cpu_set), &cpu_set);
>  	if (CHECK(err, "set_affinity", "cpu #0, err %d\n", err))
>  		goto out_destroy;
> -	/* spin the loop for a while (random high number) */
> -	for (i = 0; i < 1000000; ++i)
> +
> +	/* Spin the loop for a while by using a high iteration count, and by
> +	 * checking whether the specific run count marker has been explicitly
> +	 * incremented at least once by the backing perf_event BPF program.
> +	 */
> +	for (i = 0; i < 100000000 && !*(volatile int *)&skel->bss->run_cnt; ++i)
>  		++j;
>  
>  	test_perf_branches__detach(skel);
> diff --git a/tools/testing/selftests/bpf/progs/test_perf_branches.c b/tools/testing/selftests/bpf/progs/test_perf_branches.c
> index a1ccc831c882..05ac9410cd68 100644
> --- a/tools/testing/selftests/bpf/progs/test_perf_branches.c
> +++ b/tools/testing/selftests/bpf/progs/test_perf_branches.c
> @@ -8,6 +8,7 @@
>  #include <bpf/bpf_tracing.h>
>  
>  int valid = 0;
> +int run_cnt = 0;
>  int required_size_out = 0;
>  int written_stack_out = 0;
>  int written_global_out = 0;
> @@ -24,6 +25,8 @@ int perf_branches(void *ctx)
>  	__u64 entries[4 * 3] = {0};
>  	int required_size, written_stack, written_global;
>  
> +	++run_cnt;
> +
>  	/* write to stack */
>  	written_stack = bpf_read_branch_records(ctx, entries, sizeof(entries), 0);
>  	/* ignore spurious events */
> -- 
> 2.52.0.rc2.455.g230fcf2819-goog
>