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

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Date:   Tue, 26 May 2020 10:07:42 +0200
From:   "Eelco Chaudron" <echaudro@...hat.com>
To:     "Andrii Nakryiko" <andrii.nakryiko@...il.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 26 May 2020, at 7:29, Andrii Nakryiko wrote:

> 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?

Just coincidence, I was needing a function to flush the remaining ring 
entries, as I was using a larger wakeup_events value.
Initially, I called the function ring_buffer_flush(), but once I noticed 
your patch I renamed it :)

>> - 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.

Ack, will fix in v2

>> +
>> +       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].

Agreed, was just adding some safety checks, but in the constantly poll 
mode this is a lot of overhead. Will remover in v2.

>> +
>> +       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.

Mimicked the behavior from other functions, however just to be safe I 
split it up.

>> +               int err;
>
> nit: declare it together with "i" above, similar to how
> perf_buffer__poll does it

Put it down here as it’s only used in the context of the for loop, but 
will move it up in the v2.

>> +               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;
>>

Thanks for the review, will send out a v2 soon.