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Date: Tue, 2 Apr 2024 18:40:09 +0100
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Subject: Re: [RFC PATCH bpf-next 0/3] bpf: freeze a task cgroup from bpf
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
 Tejun Heo <tj@kernel.org>
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>
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From: Djalal Harouni <tixxdz@gmail.com>
In-Reply-To: <CAADnVQ+WmaPG1WOaSDbjxNPVzVape_JfG_CNSRy188ni076Mog@mail.gmail.com>
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Hello,

On 3/30/24 00:04, Alexei Starovoitov wrote:
> On Fri, Mar 29, 2024 at 2:39 PM Tejun Heo <tj@kernel.org> wrote:
>>
>> 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?
> 
> And helpers like cgroup_freeze_write() will be refactored
> to take kernfs_node directly instead of kernfs_open_file?
> Makes sense to me.
> Sounds like a minimal amount of changes and flexible enough.

Thank you Alexei, Tejun for the feedback. Will try to get back with a v2.

One particular thing is the kernfs_open_file->mutex nests outside of the
refcounting of kernfs_node,  let's see.

Thanks!