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

[Daniel Xu]

On Tue, Sep 10, 2024 at 11:36:36AM GMT, Alexei Starovoitov wrote:
> 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.

I'm actually unsure the difference, still. But all the other tests in
the file were using lskel so I just copy/pasted.

> 
> Also pls split selftest into a separate patch.

Ack.

> 
> >
> >  # 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.

Ack.

> 
> pw-bot: cr