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Sender: Tejun Heo <htejun@gmail.com>
Date: Fri, 29 Mar 2024 11:39:17 -1000
From: Tejun Heo <tj@kernel.org>
To: Djalal Harouni <tixxdz@gmail.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Zefan Li <lizefan.x@bytedance.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Alexei Starovoitov <ast@kernel.org>,
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	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>,
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Subject: Re: [RFC PATCH bpf-next 0/3] bpf: freeze a task cgroup from bpf
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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