Received-SPF: pass (google.com: domain of linux-kernel+bounces-125506-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Precedence: bulk
MIME-Version: 1.0
References: <20240327-ccb56fc7a6e80136db80876c@djalal> <20240327225334.58474-1-tixxdz@gmail.com>
 <ZgWnPZtwBYfHEFzf@slm.duckdns.org> <CAADnVQK6BUGZFCATD8Ejcfob5sKK-b8HUD_4o8Q6s9FM72L4iQ@mail.gmail.com>
 <ZgWv19ySvoACAll4@slm.duckdns.org> <CAADnVQLhWDcX-7XCdo-W=jthU=9iPqODwrE6c9fvU8sfAJ5ARg@mail.gmail.com>
 <ZgXMww9kJiKi4Vmd@slm.duckdns.org> <CAADnVQK970_Nx3918V41ue031RkGs+WsteOAm6EJOY7oSwzS1A@mail.gmail.com>
 <ZgXallkHApJC-adM@slm.duckdns.org> <bcb084ae-c934-4eba-aadd-95bbec2a63cb@gmail.com>
 <Zgc1BZnYCS9OSSTw@slm.duckdns.org>
In-Reply-To: <Zgc1BZnYCS9OSSTw@slm.duckdns.org>
From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date: Fri, 29 Mar 2024 16:04:53 -0700
Message-ID: <CAADnVQ+WmaPG1WOaSDbjxNPVzVape_JfG_CNSRy188ni076Mog@mail.gmail.com>
Subject: Re: [RFC PATCH bpf-next 0/3] bpf: freeze a task cgroup from bpf
To: Tejun Heo <tj@kernel.org>
Cc: Djalal Harouni <tixxdz@gmail.com>, 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>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Fri, Mar 29, 2024 at 2:39=E2=80=AFPM 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, cha=
r *buf);
>
> So, about the same, but takes kernfs_node directory instead of cgroup. Th=
is
> 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.