[PATCH bpf-next] selftests/bpf: Report the real error from libarena parallel workers

[PATCH bpf-next] selftests/bpf: Report the real error from libarena parallel workers

[Jiayuan Chen]

Several CI runs failed in the libarena parallel tests with -4 (-EINTR) [1],
which says nothing about what actually went wrong.

Two workers can fail like this:

  worker 1: gives up, e.g. the rendezvous times out, sets test_abort and
            returns its own error (-ETIMEDOUT)
  worker 2: sees test_abort and returns -EINTR

-EINTR only means "someone else already gave up", so it carries no
information. Which of the two gets reported depends on the order
pthread_join() collects them, because

	err = err ?: (long)thread_ret;

keeps the first non-zero value and drops the rest. When the -EINTR worker
comes first, the error describing the actual failure is lost.

Let any other error win over -EINTR, and log each worker that returned an
error.

It is still unclear whether the timeouts come from CI load or from a
problem in the test itself. Report the error accurately first, so the next
failure can be diagnosed.

[1]:
https://github.com/kernel-patches/bpf/actions/runs/29867905253/job/88764463566
https://github.com/kernel-patches/bpf/actions/runs/29878191901/job/88794845824

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 .../testing/selftests/bpf/prog_tests/libarena.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/libarena.c b/tools/testing/selftests/bpf/prog_tests/libarena.c
index df7e4b8dc394..65743920ff69 100644
--- a/tools/testing/selftests/bpf/prog_tests/libarena.c
+++ b/tools/testing/selftests/bpf/prog_tests/libarena.c
@@ -112,13 +112,28 @@ static int run_libarena_parallel_test_workers(struct libarena *skel,
 
 
 	for (i = 0; i < nthreads; i++) {
+		int worker_err;
+
 		ret = pthread_join(threads[i], &thread_ret);
 		if (!ASSERT_OK(ret, "pthread_join")) {
 			err = err ?: ret;
 			continue;
 		}
 
-		err = err ?: (long)thread_ret;
+		worker_err = (long)thread_ret;
+		if (!worker_err)
+			continue;
+
+		/*
+		 * A worker that bails out because another one already gave up
+		 * reports -EINTR. That is collateral damage, so let any other
+		 * error win: it tells us what actually went wrong.
+		 */
+		if (!err || err == -EINTR)
+			err = worker_err;
+
+		fprintf(stdout, "%.*s__%d returned %d\n", (int)prefixlen, name,
+			i, worker_err);
 	}
 
 	free(threads);
-- 
2.43.0

Re: [PATCH bpf-next] selftests/bpf: Report the real error from libarena parallel workers

[Emil Tsalapatis]

On Wed Jul 22, 2026 at 2:47 AM EDT, Jiayuan Chen wrote:
> Several CI runs failed in the libarena parallel tests with -4 (-EINTR) [1],
> which says nothing about what actually went wrong.
>
> Two workers can fail like this:
>
>   worker 1: gives up, e.g. the rendezvous times out, sets test_abort and
>             returns its own error (-ETIMEDOUT)
>   worker 2: sees test_abort and returns -EINTR
>
> -EINTR only means "someone else already gave up", so it carries no
> information. Which of the two gets reported depends on the order
> pthread_join() collects them, because
>
> 	err = err ?: (long)thread_ret;
>
> keeps the first non-zero value and drops the rest. When the -EINTR worker
> comes first, the error describing the actual failure is lost.
>
> Let any other error win over -EINTR, and log each worker that returned an
> error.
>
> It is still unclear whether the timeouts come from CI load or from a
> problem in the test itself. Report the error accurately first, so the next
> failure can be diagnosed.
>
> [1]:
> https://github.com/kernel-patches/bpf/actions/runs/29867905253/job/88764463566
> https://github.com/kernel-patches/bpf/actions/runs/29878191901/job/88794845824

Overall change makes sense, a couple things.

I'll also look into the error itself.

>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  .../testing/selftests/bpf/prog_tests/libarena.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/libarena.c b/tools/testing/selftests/bpf/prog_tests/libarena.c
> index df7e4b8dc394..65743920ff69 100644
> --- a/tools/testing/selftests/bpf/prog_tests/libarena.c
> +++ b/tools/testing/selftests/bpf/prog_tests/libarena.c
> @@ -112,13 +112,28 @@ static int run_libarena_parallel_test_workers(struct libarena *skel,
>  
>  
>  	for (i = 0; i < nthreads; i++) {
> +		int worker_err;
> +

Let's hoist this definition up to the top of the function.

>  		ret = pthread_join(threads[i], &thread_ret);
>  		if (!ASSERT_OK(ret, "pthread_join")) {
>  			err = err ?: ret;
>  			continue;
>  		}
>  
> -		err = err ?: (long)thread_ret;
> +		worker_err = (long)thread_ret;
> +		if (!worker_err)
> +			continue;
> +
If we are ignoring -EINTR errors from workers then let's skip the
printing for them altogether, otherwise below we keep printing the
"main" error for every interrupted thread.

> +		/*
> +		 * A worker that bails out because another one already gave up
> +		 * reports -EINTR. That is collateral damage, so let any other
> +		 * error win: it tells us what actually went wrong.

Capitalize "it"

> +		 */
> +		if (!err || err == -EINTR)
> +			err = worker_err;
> +
> +		fprintf(stdout, "%.*s__%d returned %d\n", (int)prefixlen, name,
> +			i, worker_err);

>  	}
>  
>  	free(threads);