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

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Date:   Mon, 6 Jun 2022 14:32:22 +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 Fri, Jun 03, 2022 at 12:52:27PM -0700, Yosry Ahmed <yosryahmed@...gle.com> wrote:
> Good catch. I get confused between cgrp->subsys and
> task->cgroups->subsys sometimes because of different fallback
> behavior. IIUC cgrp->subsys should have NULL if the memory controller
> is not enabled (no nearest ancestor fallback), and hence I can use
> memory_subsys_enabled() that I defined just above task_memcg() to test
> for this (I have no idea why I am not already using it here). Is my
> understanding correct?

You're correct, css_set (task->cgroups) has a css (memcg) always defined
(be it root only (or even a css from v1 hierarchy but that should not
relevant here)). A particular cgroup can have the css set to NULL.

When I think about your stats collecting example now, task_memcg() looks
more suitable to achieve proper hierarchical counting in the end (IOW
you'd lose info from tasks who don't reside in memcg-enabled leaf).

(It's just that task_memcg won't return NULL. Unless the kernel is
compiled without memcg support completely, which makes me think how do
the config-dependent values propagate to BPF programs?)

Thanks,
Michal