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

[Pedro Tammela <pctammela@gmail.com>]

Em qua., 7 de abr. de 2021 às 15:31, Andrii Nakryiko
<andrii.nakryiko@gmail.com> escreveu:
>
> 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.

From my own experience, the documentation has been a very unreliable
source, to the point that I usually jump to the code first rather than
to the documentation nowadays[1].
Tests, samples and projects have always been my source of truth and we
are already lacking a bit on those as well. For instance, the samples
directory contains programs that are very outdated (I didn't check if
they are still functional).
I think macros like these will be present in most of the project
dealing with batched operations and as a daily user of libbpf I don't
see how this could not be offered by libbpf as a standardized way to
declare percpu types.

[1] So batched operations were introduced a little bit over a 1 year
ago and yet the only reference I had for it was the selftests. The
documentation is on my TODO list, but that's just because I have to
deal with it daily.

>
> 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?

Why not do both?

Provide a standardized way to declare a percpu value with examples and
a good documentation with examples.
Let the user decide what is best for his use case.

>
> >
> > 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)?

OK. Makes sense.

>
> > -       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
> >