Re: [PATCH bpf-next] bpf: ringbuf: Support consuming BPF_MAP_TYPE_RINGBUF from prog

[Alexei Starovoitov]

On Mon, Sep 9, 2024 at 5:55 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
>
> Right now there exists prog produce / userspace consume and userspace
> produce / prog consume support. But it is also useful to have prog
> produce / prog consume.
>
> For example, we want to track the latency overhead of cpumap in
> production. Since we need to store enqueue timestamps somewhere and
> cpumap is MPSC, we need an MPSC data structure to shadow cpumap. BPF
> ringbuf is such a data structure. Rather than reimplement (possibly
> poorly) a custom ringbuffer in BPF, extend the existing interface to
> allow the final quadrant of ringbuf usecases to be filled. Note we
> ignore userspace to userspace use case - there is no need to involve
> kernel for that.
>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> ---
>  kernel/bpf/verifier.c                         |  6 +-
>  tools/testing/selftests/bpf/Makefile          |  3 +-
>  .../selftests/bpf/prog_tests/ringbuf.c        | 50 +++++++++++++++
>  .../bpf/progs/test_ringbuf_bpf_to_bpf.c       | 64 +++++++++++++++++++
>  4 files changed, 120 insertions(+), 3 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/progs/test_ringbuf_bpf_to_bpf.c
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 53d0556fbbf3..56bfe559f228 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9142,7 +9142,8 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
>                     func_id != BPF_FUNC_ringbuf_query &&
>                     func_id != BPF_FUNC_ringbuf_reserve_dynptr &&
>                     func_id != BPF_FUNC_ringbuf_submit_dynptr &&
> -                   func_id != BPF_FUNC_ringbuf_discard_dynptr)
> +                   func_id != BPF_FUNC_ringbuf_discard_dynptr &&
> +                   func_id != BPF_FUNC_user_ringbuf_drain)
>                         goto error;
>                 break;
>         case BPF_MAP_TYPE_USER_RINGBUF:
> @@ -9276,7 +9277,8 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
>                         goto error;
>                 break;
>         case BPF_FUNC_user_ringbuf_drain:
> -               if (map->map_type != BPF_MAP_TYPE_USER_RINGBUF)
> +               if (map->map_type != BPF_MAP_TYPE_USER_RINGBUF &&
> +                   map->map_type != BPF_MAP_TYPE_RINGBUF)
>                         goto error;

I think it should work.

Andrii,

do you see any issues with such use?

>                 break;
>         case BPF_FUNC_get_stackid:
> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> index 9905e3739dd0..233109843d4d 100644
> --- a/tools/testing/selftests/bpf/Makefile
> +++ b/tools/testing/selftests/bpf/Makefile
> @@ -503,7 +503,8 @@ LINKED_SKELS := test_static_linked.skel.h linked_funcs.skel.h               \
>  LSKELS := fentry_test.c fexit_test.c fexit_sleep.c atomics.c           \
>         trace_printk.c trace_vprintk.c map_ptr_kern.c                   \
>         core_kern.c core_kern_overflow.c test_ringbuf.c                 \
> -       test_ringbuf_n.c test_ringbuf_map_key.c test_ringbuf_write.c
> +       test_ringbuf_n.c test_ringbuf_map_key.c test_ringbuf_write.c    \
> +       test_ringbuf_bpf_to_bpf.c

Do you need it to be lskel ?

Regular skels are either to debug.

Also pls split selftest into a separate patch.

>
>  # Generate both light skeleton and libbpf skeleton for these
>  LSKELS_EXTRA := test_ksyms_module.c test_ksyms_weak.c kfunc_call_test.c \
> diff --git a/tools/testing/selftests/bpf/prog_tests/ringbuf.c b/tools/testing/selftests/bpf/prog_tests/ringbuf.c
> index da430df45aa4..3e7955573ac5 100644
> --- a/tools/testing/selftests/bpf/prog_tests/ringbuf.c
> +++ b/tools/testing/selftests/bpf/prog_tests/ringbuf.c
> @@ -17,6 +17,7 @@
>  #include "test_ringbuf_n.lskel.h"
>  #include "test_ringbuf_map_key.lskel.h"
>  #include "test_ringbuf_write.lskel.h"
> +#include "test_ringbuf_bpf_to_bpf.lskel.h"
>
>  #define EDONE 7777
>
> @@ -497,6 +498,53 @@ static void ringbuf_map_key_subtest(void)
>         test_ringbuf_map_key_lskel__destroy(skel_map_key);
>  }
>
> +static void ringbuf_bpf_to_bpf_subtest(void)
> +{
> +       struct test_ringbuf_bpf_to_bpf_lskel *skel;
> +       int err, i;
> +
> +       skel = test_ringbuf_bpf_to_bpf_lskel__open();
> +       if (!ASSERT_OK_PTR(skel, "test_ringbuf_bpf_to_bpf_lskel__open"))
> +               return;
> +
> +       skel->maps.ringbuf.max_entries = getpagesize();
> +       skel->bss->pid = getpid();
> +
> +       err = test_ringbuf_bpf_to_bpf_lskel__load(skel);
> +       if (!ASSERT_OK(err, "test_ringbuf_bpf_to_bpf_lskel__load"))
> +               goto cleanup;
> +
> +       ringbuf = ring_buffer__new(skel->maps.ringbuf.map_fd, NULL, NULL, NULL);
> +       if (!ASSERT_OK_PTR(ringbuf, "ring_buffer__new"))
> +               goto cleanup;
> +
> +       err = test_ringbuf_bpf_to_bpf_lskel__attach(skel);
> +       if (!ASSERT_OK(err, "test_ringbuf_bpf_to_bpf_lskel__attach"))
> +               goto cleanup_ringbuf;
> +
> +       /* Produce N_SAMPLES samples in the ring buffer by calling getpid() */
> +       skel->bss->value = SAMPLE_VALUE;
> +       for (i = 0; i < N_SAMPLES; i++)
> +               syscall(__NR_getpgid);
> +
> +       /* Trigger bpf-side consumption */
> +       syscall(__NR_prctl);

This might conflict with other selftests that run in parallel.

Just load the skel and bpf_prog_run(prog_fd).
No need to attach anywhere in the kernel.

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