netdev - Re: [PATCH bpf-next v1 5/5] bpf: add a selftest for cgroup hierarchical stats collection

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Date:   Tue, 7 Jun 2022 14:12:37 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shuah Khan <shuah@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Michal Hocko <mhocko@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH bpf-next v1 5/5] bpf: add a selftest for cgroup
 hierarchical stats collection

On Mon, Jun 06, 2022 at 12:41:06PM -0700, Yosry Ahmed <yosryahmed@...gle.com> wrote:
> I don't know if there is a standard way to handle this, but I think
> you should know the configs of your kernel when you are loading a bpf
> program?

Isn't this one of purposes of BTF? (I don't know, I'm genuinely asking.)

> If the CONFIG_CGROUPS=1 but CONFIG_MEMCG=0 I think everything will
> work normally except that task_memcg() will always return NULL so no
> stats will be collected, which makes sense.

I was not able to track down what is the include chain to
tools/testing/selftests/bpf/progs/cgroup_vmscan.c, i.e. how is the enum
value memory_cgrp_id defined.

(A custom kernel module build requires target kernel's header files, I
could understand that compiling a BPF program requires them likewise and
that's how this could work.
Although, it goes against my undestanding of the CO-RE principle.)

> There will be some overhead to running bpf programs that will always
> do nothing, but I would argue that it's the userspace's fault here for
> loading bpf programs on a non-compatible kernel.

Yeah, running an empty program is non-issue in my eyes, I was rather
considering whether the program uses proper offsets.

Michal