linux-kernel - Re: [PATCH] selftests/bpf: simplify cgroup_hierarchical

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Date:   Mon, 29 Aug 2022 13:08:36 -0700
From:   Hao Luo <haoluo@...gle.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Mykola Lysenko <mykolal@...com>, Song Liu <song@...nel.org>,
        Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selftests/bpf: simplify cgroup_hierarchical_stats selftest

On Fri, Aug 26, 2022 at 4:06 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> The cgroup_hierarchical_stats selftest is complicated. It has to be,
> because it tests an entire workflow of recording, aggregating, and
> dumping cgroup stats. However, some of the complexity is unnecessary.
> The test now enables the memory controller in a cgroup hierarchy, invokes
> reclaim, measure reclaim time, THEN uses that reclaim time to test the
> stats collection and aggregation. We don't need to use such a
> complicated stat, as the context in which the stat is collected is
> orthogonal.
>
> Simplify the test by using a simple stat instead of reclaim time, the
> total number of times a process has ever entered a cgroup. This makes
> the test simpler and removes the dependency on the memory controller and
> the memory reclaim interface.
>
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> ---

Yosry, please tag the patch with the repo it should be applied on:
bpf-next or bpf.

>
> When the test failed on Alexei's setup because the memory controller was
> not enabled I realized this is an unnecessary dependency for the test,
> which inspired this patch :) I am not sure if this prompt a Fixes tag as
> the test wasn't broken.
>
> ---
>  .../prog_tests/cgroup_hierarchical_stats.c    | 157 ++++++---------
>  .../bpf/progs/cgroup_hierarchical_stats.c     | 181 ++++++------------
>  2 files changed, 118 insertions(+), 220 deletions(-)
>
[...]
> diff --git a/tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c b/tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c
> index 8ab4253a1592..c74362854948 100644
> --- a/tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c
> +++ b/tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c
> @@ -1,7 +1,5 @@
>  // SPDX-License-Identifier: GPL-2.0-only
>  /*
> - * Functions to manage eBPF programs attached to cgroup subsystems
> - *

Please also add comments here explaining what the programs in this file do.

>   * Copyright 2022 Google LLC.
>   */
[...]
>
> -SEC("tp_btf/mm_vmscan_memcg_reclaim_begin")
> -int BPF_PROG(vmscan_start, int order, gfp_t gfp_flags)
> +SEC("fentry/cgroup_attach_task")

Can we select an attachpoint that is more stable? It seems
'cgroup_attach_task' is an internal helper function in cgroup, and its
signature can change. I'd prefer using those commonly used tracepoints
and EXPORT'ed functions. IMHO their interfaces are more stable.

> +int BPF_PROG(counter, struct cgroup *dst_cgrp, struct task_struct *leader,
> +            bool threadgroup)
>  {
> -       struct task_struct *task = bpf_get_current_task_btf();
> -       __u64 *start_time_ptr;
> -
> -       start_time_ptr = bpf_task_storage_get(&vmscan_start_time, task, 0,
> -                                             BPF_LOCAL_STORAGE_GET_F_CREATE);
> -       if (start_time_ptr)
> -               *start_time_ptr = bpf_ktime_get_ns();
> -       return 0;
> -}
[...]
>  }
> --
> 2.37.2.672.g94769d06f0-goog
>