Re: [PATCH] tools/sched_ext: scx_flatcg: Document cgroup v2 requirement

[Tejun Heo <tj@kernel.org>]

On Wed, Jun 03, 2026 at 05:01:06PM +0800, Liang Luo wrote:
> The BPF program relies on cgroup v2-specific features - specifically
> BPF_MAP_TYPE_CGRP_STORAGE and cgroup v2 kfuncs (bpf_cgroup_ancestor,
> bpf_cgroup_from_id, scx_bpf_task_cgroup, ...), none of which are
> available on cgroup v1. The current file header describes the
> algorithm but does not make this runtime requirement explicit, so
> users running on a cgroup v1-only system get a confusing BPF load
> error with no clear pointer to the cause.

This doesn't look accurate on current kernels. BPF_MAP_TYPE_CGRP_STORAGE is
registered unconditionally and its fd access goes through
cgroup_v1v2_get_from_fd(), and bpf_cgroup_ancestor(), bpf_cgroup_from_id()
and scx_bpf_task_cgroup() all take a plain struct cgroup with no v1/v2
gating. The program should load fine on cgroup1.

Did you actually hit a load failure, and on which kernel? cgrp_storage's
cgroup1 access was only enabled in v6.8 by 73d9eb340d2b ("bpf: Enable
bpf_cgrp_storage for cgroup1 non-attach case"). Before that it was cgroup2
only, which would explain a failure on an older kernel.

Thanks.

--
tejun