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 11:39:17 -1000
From: Tejun Heo <tj@...nel.org>
To: Djalal Harouni <tixxdz@...il.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
	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,

On Fri, Mar 29, 2024 at 02:22:28PM +0100, Djalal Harouni wrote:
> 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.

I've thought about it a bit and I wonder whether a better way to do this is
implementing this at the kernfs layer. Something like (hopefully with a
better name):

 s32 bpf_kernfs_knob_write(struct kernfs_node *dir, const char *knob, char *buf);

So, about the same, but takes kernfs_node directory instead of cgroup. This
would make the interface useful for accessing sysfs knobs too which use
similar conventions. For cgroup, @dir is just cgrp->kn and for sysfs it'd be
kobj->sd. This way we can avoid the internal object -> path -> internal
object ping-poinging while keeping the interface a lot more generic. What do
you think?

Thanks.

-- 
tejun