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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Mon, 6 Jun 2022 12:41:06 -0700
Message-ID: <CAJD7tkbi7Gnnf4NiUt-J61G7185NsRcySvP6qOQsFKMou7qZJg@mail.gmail.com>
Subject: Re: [PATCH bpf-next v1 5/5] bpf: add a selftest for cgroup
 hierarchical stats collection
To:     =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Hao Luo <haoluo@google.com>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shuah Khan <shuah@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Michal Hocko <mhocko@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        David Rientjes <rientjes@google.com>,
        Greg Thelen <gthelen@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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On Mon, Jun 6, 2022 at 5:32 AM Michal Koutn=C3=BD <mkoutny@suse.com> wrote:
>
> On Fri, Jun 03, 2022 at 12:52:27PM -0700, Yosry Ahmed <yosryahmed@google.=
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).

I guess it depends on how userspace reasons about this, and whether or
not you want to collect stats from leaves that don't reside in a
memcg-enabled leaf. I will go through all the memcg-enabled checks and
make sure they make sense and are consistent, maybe add some comments
to make the userspace policy here clear.

>
> (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?)

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? In this particular case, if CONFIG_CGROUPS=3D0 then the bpf
programs will not even load due to lack of hook points or kfuncs won't
exist. If the CONFIG_CGROUPS=3D1 but CONFIG_MEMCG=3D0 I think everything
will work normally except that task_memcg() will always return NULL so
no stats will be collected, which makes sense. 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.

>
> Thanks,
> Michal