linux-kernel - Re: [PATCH v3] selftests/bpf: Add ring_buffer__consume

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Date: Thu, 25 Apr 2024 11:47:07 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Andrea Righi <andrea.righi@...onical.com>
Cc: Andrii Nakryiko <andrii@...nel.org>, Eduard Zingerman <eddyz87@...il.com>, 
	Mykola Lysenko <mykolal@...com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org, 
	bpf@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v3] selftests/bpf: Add ring_buffer__consume_n test.

On Thu, Apr 25, 2024 at 7:06 AM Andrea Righi <andrea.righi@...onical.com> wrote:
>
> Add a testcase for the ring_buffer__consume_n() API.
>
> The test produces multiple samples in a ring buffer, using a
> sys_getpid() fentry prog, and consumes them from user-space in batches,
> rather than consuming all of them greedily, like ring_buffer__consume()
> does.
>
> Link: https://lore.kernel.org/lkml/CAEf4BzaR4zqUpDmj44KNLdpJ=Tpa97GrvzuzVNO5nM6b7oWd1w@mail.gmail.com
> Signed-off-by: Andrea Righi <andrea.righi@...onical.com>
> ---
>  tools/testing/selftests/bpf/Makefile          |  2 +-
>  .../selftests/bpf/prog_tests/ringbuf.c        | 64 +++++++++++++++++++
>  .../selftests/bpf/progs/test_ringbuf_n.c      | 47 ++++++++++++++
>  3 files changed, 112 insertions(+), 1 deletion(-)
>  create mode 100644 tools/testing/selftests/bpf/progs/test_ringbuf_n.c
>
> ChangeLog v2 -> v3:
>  - move skel_n inside ringbuf_n_subtest()
>
> ChangeLog v1 -> v2:
>  - replace CHECK() with ASSERT_EQ()
>  - fix skel -> skel_n
>  - drop unused "seq" field from struct sample
>

[...]

> +       /* Produce N_TOT_SAMPLES samples in the ring buffer by calling getpid() */
> +       skel_n->bss->value = SAMPLE_VALUE;
> +       for (i = 0; i < N_TOT_SAMPLES; i++)
> +               syscall(__NR_getpgid);
> +
> +       /* Consume all samples from the ring buffer in batches of N_SAMPLES */
> +       for (i = 0; i < N_TOT_SAMPLES; i += err) {
> +               err = ring_buffer__consume_n(ringbuf, N_SAMPLES);
> +               ASSERT_EQ(err, N_SAMPLES, "rb_consume");

if something goes wrong and err is < 0, we might end up with a very
long loop. I changed this to:

if (!ASSERT_EQ(...))
    goto cleanup_ringbuf;

to avoid this problem

> +       }
> +
> +cleanup_ringbuf:
> +       ring_buffer__free(ringbuf);
> +cleanup:
> +       test_ringbuf_n_lskel__destroy(skel_n);
> +}
> +

[...]