Re: [PATCH bpf-next 2/2] selftests/bpf: Add selftest for bits iter

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2024-02-23 at 10:29 +0800, Yafang Shao wrote:

[...]

> > The patch 1 looks good, but this test fails on s390.
> > 
> > read_percpu_data:FAIL:nr_cpus unexpected nr_cpus: actual 0 != expected 2
> > verify_iter_success:FAIL:read_percpu_data unexpected error: -1 (errno 95)
> > 
> > Please see CI.
> > 
> > So either add it to DENYLIST.s390x in the same commit or make it work.
> > 
> > pw-bot: cr
> 
> The reason for the failure on s390x architecture is currently unclear.
> One plausible explanation is that total_nr_cpus remains 0 when
> executing the following code:
> 
>     bpf_for_each(bits, cpu, p->cpus_ptr, total_nr_cpus)
> 
> This is despite setting total_nr_cpus to the value obtained from
> libbpf_num_possible_cpus():
> 
>     skel->bss->total_nr_cpus = libbpf_num_possible_cpus();
> 
> A potential workaround could involve using a hardcoded number of CPUs,
> such as 8192, instead of relying on total_nr_cpus. This approach might
> mitigate the issue temporarily. 

I'm sorry, but is it really necessary to deal with total number of
CPUs in a test for bit iterator?
Tbh, cpumask_iter / verify_iter_success seem to be over-complicated.
Would it be possible to reuse test_loader.c's RUN_TESTS for this feature?
It supports __retval(...) annotation, so it should be possible to:
- create a map (even a constant map) with some known data;
- peek a BPF program type that supports BPF_PROG_TEST_RUN syscall command;
- organize test BPF programs so that they create bit iterators for
  this test data and return some expected quantities (e.g. a sum),
  verified by __retval.

This should limit the amount of code on prog_tests/*.c side
to the bare minimum.