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Subject: Re: [PATCH] libbpf: Expose API to consume one ring at a time
From:   Hou Tao <houtao@huaweicloud.com>
To:     Adam Sindelar <adam@wowsignal.io>, bpf@vger.kernel.org
Cc:     Adam Sindelar <ats@fb.com>, David Vernet <void@manifault.com>,
        Brendan Jackman <jackmanb@google.com>,
        KP Singh <kpsingh@chromium.org>, linux-kernel@vger.kernel.org,
        Alexei Starovoitov <ast@kernel.org>,
        Florent Revest <revest@chromium.org>,
        Andrii Nakryiko <andrii@kernel.org>
References: <20230725162654.912897-1-adam@wowsignal.io>
 <cb844776-9045-1b69-f1db-8ef7d75815b5@huaweicloud.com>
Message-ID: <482ed32c-5650-54a5-d5bb-18b9bb03e838@huaweicloud.com>
Date:   Thu, 27 Jul 2023 14:17:12 +0800
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Hi,

On 7/27/2023 9:06 AM, Hou Tao wrote:
> Hi,
>
> On 7/26/2023 12:26 AM, Adam Sindelar wrote:
>> We already provide ring_buffer__epoll_fd to enable use of external
>> polling systems. However, the only API available to consume the ring
>> buffer is ring_buffer__consume, which always checks all rings. When
>> polling for many events, this can be wasteful.
>>
>> Signed-off-by: Adam Sindelar <adam@wowsignal.io>
>> ---
>>  tools/lib/bpf/libbpf.h  |  1 +
>>  tools/lib/bpf/ringbuf.c | 15 +++++++++++++++
>>  2 files changed, 16 insertions(+)
>>
>> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>> index 55b97b2087540..20ccc65eb3f9d 100644
>> --- a/tools/lib/bpf/libbpf.h
>> +++ b/tools/lib/bpf/libbpf.h
>> @@ -1195,6 +1195,7 @@ LIBBPF_API int ring_buffer__add(struct ring_buffer *rb, int map_fd,
>>  				ring_buffer_sample_fn sample_cb, void *ctx);
>>  LIBBPF_API int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms);
>>  LIBBPF_API int ring_buffer__consume(struct ring_buffer *rb);
>> +LIBBPF_API int ring_buffer__consume_ring(struct ring_buffer *rb, uint32_t ring_id);
>>  LIBBPF_API int ring_buffer__epoll_fd(const struct ring_buffer *rb);
>>  
>>  struct user_ring_buffer_opts {
>> diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
>> index 02199364db136..8d087bfc7d005 100644
>> --- a/tools/lib/bpf/ringbuf.c
>> +++ b/tools/lib/bpf/ringbuf.c
>> @@ -290,6 +290,21 @@ int ring_buffer__consume(struct ring_buffer *rb)
>>  	return res;
>>  }
>>  
>> +/* Consume available data from a single RINGBUF map identified by its ID.
>> + * The ring ID is returned in epoll_data by epoll_wait when called with
>> + * ring_buffer__epoll_fd.
>> + */
>> +int ring_buffer__consume_ring(struct ring_buffer *rb, uint32_t ring_id)
>> +{
>> +	struct ring *ring;
>> +
>> +	if (ring_id >= rb->ring_cnt)
>> +		return libbpf_err(-EINVAL);
>> +
>> +	ring = &rb->rings[ring_id];
>> +	return ringbuf_process_ring(ring);
> When ringbuf_process_ring() returns an error, we need to use
> libbpf_err() to set the errno accordingly.

It seems that even when ringbuf_process_ring() returns a positive
result, we also need to cap it under INT_MAX, otherwise it may be cast
into a negative error.
>> +}
>> +
>>  /* Poll for available data and consume records, if any are available.
>>   * Returns number of records consumed (or INT_MAX, whichever is less), or
>>   * negative number, if any of the registered callbacks returned error.
>
>
> .