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

From: Djalal Harouni <tixxdz@gmail.com>
To: Tejun Heo <tj@kernel.org>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Zefan Li <lizefan.x@bytedance.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
	Yonghong Song <yonghong.song@linux.dev>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
	Jiri Olsa <jolsa@kernel.org>, Mykola Lysenko <mykolal@fb.com>,
	Shuah Khan <shuah@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"open list:CONTROL GROUP (CGROUP)" <cgroups@vger.kernel.org>,
	bpf <bpf@vger.kernel.org>,
	"open list:KERNEL SELFTEST FRAMEWORK"
	<linux-kselftest@vger.kernel.org>
Subject: Re: [RFC PATCH bpf-next 0/3] bpf: freeze a task cgroup from bpf
Date: Fri, 29 Mar 2024 14:22:28 +0100	[thread overview]
Message-ID: <bcb084ae-c934-4eba-aadd-95bbec2a63cb@gmail.com> (raw)
In-Reply-To: <ZgXallkHApJC-adM@slm.duckdns.org>

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@kernel.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!