Re: [PATCH bpf-next v2 2/3] libbpf: selftests: refactor 'BPF_PERCPU_TYPE()' and 'bpf_percpu()' macros

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Apr 6, 2021 at 11:55 AM Pedro Tammela <pctammela@gmail.com> wrote:
>
> This macro was refactored out of the bpf selftests.
>
> Since percpu values are rounded up to '8' in the kernel, a careless
> user in userspace might encounter unexpected values when parsing the
> output of the batched operations.

I wonder if a user has to be more careful, though? This
BPF_PERCPU_TYPE, __bpf_percpu_align and bpf_percpu macros seem to
create just another opaque layer. It actually seems detrimental to me.

I'd rather emphasize in the documentation (e.g., in
bpf_map_lookup_elem) that all per-cpu maps are aligning values at 8
bytes, so user has to make sure that array of values provided to
bpf_map_lookup_elem() has each element size rounded up to 8.

In practice, I'd recommend users to always use __u64/__s64 when having
primitive integers in a map (they are not saving anything by using
int, it just creates an illusion of savings). Well, maybe on 32-bit
arches they would save a bit of CPU, but not on typical 64-bit
architectures. As for using structs as values, always mark them as
__attribute__((aligned(8))).

Basically, instead of obscuring the real use some more, let's clarify
and maybe even provide some examples in documentation?

>
> Now that both array and hash maps have support for batched ops in the
> percpu variant, let's provide a convenient macro to declare percpu map
> value types.
>
> Updates the tests to a "reference" usage of the new macro.
>
> Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
> ---
>  tools/lib/bpf/bpf.h                           | 10 ++++
>  tools/testing/selftests/bpf/bpf_util.h        |  7 ---
>  .../bpf/map_tests/htab_map_batch_ops.c        | 48 ++++++++++---------
>  .../selftests/bpf/prog_tests/map_init.c       |  5 +-
>  tools/testing/selftests/bpf/test_maps.c       | 16 ++++---
>  5 files changed, 46 insertions(+), 40 deletions(-)
>

[...]

> @@ -400,11 +402,11 @@ static void test_arraymap(unsigned int task, void *data)
>  static void test_arraymap_percpu(unsigned int task, void *data)
>  {
>         unsigned int nr_cpus = bpf_num_possible_cpus();
> -       BPF_DECLARE_PERCPU(long, values);
> +       pcpu_map_value_t values[nr_cpus];
>         int key, next_key, fd, i;
>
>         fd = bpf_create_map(BPF_MAP_TYPE_PERCPU_ARRAY, sizeof(key),
> -                           sizeof(bpf_percpu(values, 0)), 2, 0);
> +                           sizeof(long), 2, 0);
>         if (fd < 0) {
>                 printf("Failed to create arraymap '%s'!\n", strerror(errno));
>                 exit(1);
> @@ -459,7 +461,7 @@ static void test_arraymap_percpu(unsigned int task, void *data)
>  static void test_arraymap_percpu_many_keys(void)
>  {
>         unsigned int nr_cpus = bpf_num_possible_cpus();

This just sets a bad example for anyone using selftests as an
aspiration for their own code. bpf_num_possible_cpus() does exit(1)
internally if libbpf_num_possible_cpus() returns error. No one should
write real production code like that. So maybe let's provide a better
example instead with error handling and malloc (or perhaps alloca)?

> -       BPF_DECLARE_PERCPU(long, values);
> +       pcpu_map_value_t values[nr_cpus];
>         /* nr_keys is not too large otherwise the test stresses percpu
>          * allocator more than anything else
>          */
> @@ -467,7 +469,7 @@ static void test_arraymap_percpu_many_keys(void)
>         int key, fd, i;
>
>         fd = bpf_create_map(BPF_MAP_TYPE_PERCPU_ARRAY, sizeof(key),
> -                           sizeof(bpf_percpu(values, 0)), nr_keys, 0);
> +                           sizeof(long), nr_keys, 0);
>         if (fd < 0) {
>                 printf("Failed to create per-cpu arraymap '%s'!\n",
>                        strerror(errno));
> --
> 2.25.1
>