linux-kernel - Re: [RFC PATCH bpf-next 0/3] bpf: freeze a task cgroup from bpf

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Date: Fri, 29 Mar 2024 14:22:28 +0100
From: Djalal Harouni <tixxdz@...il.com>
To: Tejun Heo <tj@...nel.org>,
 Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Zefan Li <lizefan.x@...edance.com>, Johannes Weiner <hannes@...xchg.org>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>,
 Shuah Khan <shuah@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
 "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
 bpf <bpf@...r.kernel.org>,
 "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>
Subject: Re: [RFC PATCH bpf-next 0/3] bpf: freeze a task cgroup from bpf

Hello Tejun, Alexei,

On 3/28/24 22:01, Tejun Heo wrote:
> Hello,
> 
> On Thu, Mar 28, 2024 at 01:45:56PM -0700, Alexei Starovoitov wrote:
>> On Thu, Mar 28, 2024 at 1:02 PM Tejun Heo <tj@...nel.org> wrote:
>>>
>>> There's also cgroup.kill which would be useful for similar use cases. We can
>>> add interface for both but idk. Let's say we have something like the
>>> following (pardon the bad naming):
>>>

Yes having the cgroup.kill from bpf would be useful!


>>>   bpf_cgroup_knob_write(struct cgroup *cgrp, char *filename, char *buf)
>>>
>>> Would that work? I'm not necessarily in love with the idea or against adding
>>> separate helpers but the duplication still bothers me a bit.
>>
>> I liked it.
>> So filename will be one of cgroup_base_files[].name ?
>> We probably don't want psi or cgroup1_base_files in there.
> 
> Would it matter? If the user has root perm, they can do whatever with the
> files anyway, so I'm not sure why we'd restrict any specific knob. Maybe we
> wanna make sure @filename doesn't include '/'? Or is it that you don't want
> to go through the usual file name look up?

It would be easy at least for me if I just start with cgroupv2 and
ensure that it has same available filenames as if we go through kernfs.
Not a root cgroup node and maybe only freeze and kill for now that are
part of cgroup_base_files.

So if I get it right, somehow like what I did but we endup with:

In bpf, cgroup was already acquired.

bpf_cgroup_knob_write(cgroup, "freeze", buf)
|_ parse params -> lock cgroup_mutex -> cgroup_freeze() -> unlock


cgroup_freeze_write(struct kernfs_open_file *of, char *buf,...)
|_ parse params -> cgroup_ref++ -> krnfs_active_ref--  ->
     -> lock cgroup_mutex -> cgroup_freeze() -> unlock + krnfs++ ...

Please let me know if I missed something.

Thanks!