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 16:04:53 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Tejun Heo <tj@...nel.org>
Cc: Djalal Harouni <tixxdz@...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

On Fri, Mar 29, 2024 at 2:39 PM Tejun Heo <tj@...nel.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.