Re: [PATCH] sched_ext: allow scx_bpf_task_cgroup() in ops.dispatch()

[Tejun Heo <tj@kernel.org>]

Hello,

On Mon, Sep 15, 2025 at 03:52:36PM +0900, Changwoo Min wrote:
...
> Fix this by adding the prev task to scx.kf_tasks so that task-related
> BPF helpers such as scx_bpf_task_cgroup() can be called safely. Since
> the SCX_CALL_OP_TASK family assumes the first argument is the task,
> introduce a new SCX_CALL_OP_TASK_ANY macro without that restriction.
> Also update __SCX_KF_TERMINAL to include SCX_KF_DISPATCH.

I'm not sure this is safe tho. ops.dispatch() can release the rq lock it's
holding to migrate tasks across rq's, which means that other operations can
nest inside - ie. there can be an irq which triggers ops.enqueue() while
ops.dispatch() is in progress. That can in turn overwrite
current->scx.kf_tasks[].

I wonder whether a better approach would be tracking cgroup membership from
BPF side. ops.init_task() tells you the initial cgroup it's joining and if
the task later moves to another cgroup, ops.cgroup_move() will be invoked.
Would that work?

Thanks.

-- 
tejun