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

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Message-ID: <CAJD7tkYa3u52c77cnRxZ6D_4u5fkDG545r5a9SdK3Ys9Uuorig@mail.gmail.com>
Date:   Tue, 7 Jun 2022 10:43:35 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Michal Koutný <mkoutny@...e.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 Tue, Jun 7, 2022 at 5:12 AM Michal Koutný <mkoutny@...e.com> wrote:
>
> 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.

memory_cgrp_id is defined in "vmlinux.h" (generated from BTF) which is
included through "bpf_iter.h". If the kernel is not compiled with
CONFIG_MEMCG then this enum value will not be defined and the bpf prog
should not compile.

>
> (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
>