netdev - Re: [PATCH bpf-next] libbpf: add API to consume the perf ring buffer content

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Date:   Mon, 25 May 2020 22:29:58 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Eelco Chaudron <echaudro@...hat.com>
Cc:     bpf <bpf@...r.kernel.org>, "David S. Miller" <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>, Andrii Nakryiko <andriin@...com>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH bpf-next] libbpf: add API to consume the perf ring buffer content

On Mon, May 25, 2020 at 2:01 PM Eelco Chaudron <echaudro@...hat.com> wrote:
>
> This new API, perf_buffer__consume, can be used as follows:

I wonder, was it inspired by yet-to-be committed
ring_buffer__consume() or it's just a coincidence?

> - When you have a perf ring where wakeup_events is higher than 1,
>   and you have remaining data in the rings you would like to pull
>   out on exit (or maybe based on a timeout).
> - For low latency cases where you burn a CPU that constantly polls
>   the queues.
>
> Signed-off-by: Eelco Chaudron <echaudro@...hat.com>
> ---
>  tools/lib/bpf/libbpf.c   |   23 +++++++++++++++++++++++
>  tools/lib/bpf/libbpf.h   |    1 +
>  tools/lib/bpf/libbpf.map |    1 +
>  3 files changed, 25 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index fa04cbe547ed..cbef3dac7507 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -8456,6 +8456,29 @@ int perf_buffer__poll(struct perf_buffer *pb, int timeout_ms)
>         return cnt < 0 ? -errno : cnt;
>  }
>
> +int perf_buffer__consume(struct perf_buffer *pb)
> +{
> +       int i;
> +
> +       if (!pb)
> +               return -EINVAL;

we don't check this in perf_buffer__poll, IMO, checking this in every
"method" is an overkill.

> +
> +       if (!pb->cpu_bufs)
> +               return 0;

no need to check. It's either non-NULL for valid perf_buffer, or
calloc could return NULL if pb->cpu_cnt is zero (not sure it's
possible, but still), but then loop below will never access
pb->cpu_bufs[i].

> +
> +       for (i = 0; i < pb->cpu_cnt && pb->cpu_bufs[i]; i++) {

I think pb->cpu_bufs[i] check is wrong, it will stop iteration
prematurely if cpu_bufs are sparsely populated. So move check inside
and continue loop if NULL.

> +               int err;

nit: declare it together with "i" above, similar to how
perf_buffer__poll does it

> +               struct perf_cpu_buf *cpu_buf = pb->cpu_bufs[i];
> +
> +               err = perf_buffer__process_records(pb, cpu_buf);
> +               if (err) {
> +                       pr_warn("error while processing records: %d\n", err);
> +                       return err;
> +               }
> +       }
> +       return 0;
> +}
> +
>  struct bpf_prog_info_array_desc {
>         int     array_offset;   /* e.g. offset of jited_prog_insns */
>         int     count_offset;   /* e.g. offset of jited_prog_len */
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 8ea69558f0a8..1e2e399a5f2c 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -533,6 +533,7 @@ perf_buffer__new_raw(int map_fd, size_t page_cnt,
>
>  LIBBPF_API void perf_buffer__free(struct perf_buffer *pb);
>  LIBBPF_API int perf_buffer__poll(struct perf_buffer *pb, int timeout_ms);
> +LIBBPF_API int perf_buffer__consume(struct perf_buffer *pb);
>
>  typedef enum bpf_perf_event_ret
>         (*bpf_perf_event_print_t)(struct perf_event_header *hdr,
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 0133d469d30b..381a7342ecfc 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -262,4 +262,5 @@ LIBBPF_0.0.9 {
>                 bpf_link_get_fd_by_id;
>                 bpf_link_get_next_id;
>                 bpf_program__attach_iter;
> +               perf_buffer__consume;
>  } LIBBPF_0.0.8;
>