Re: [PATCH sched_ext/for-6.18] sched_ext: Acquire task reference in scx_bpf_cpu_curr()

[Tejun Heo <tj@kernel.org>]

Hello, Andrea.

On Tue, Sep 09, 2025 at 09:57:09PM +0200, Andrea Righi wrote:
> scx_bpf_cpu_curr() has been introduced to retrieve the current task of a
> given runqueue, allowing schedulers to interact with that task.
> 
> The kfunc assumes that it is always called in an RCU context, but this
> is not always guaranteed and some BPF schedulers can trigger the
> following warning:
> 
>   WARNING: suspicious RCU usage
>   sched_ext: BPF scheduler "cosmos_1.0.2_gd0e71ca_x86_64_unknown_linux_gnu_debug" enabled
>   6.17.0-rc1 #1-NixOS Not tainted
>   -----------------------------
>   kernel/sched/ext.c:6415 suspicious rcu_dereference_check() usage!
> 
> The correct behavior is to acquire a reference to the returned task, so
> the scheduler can safely access it and then release it with
> bpf_task_release().
> 
> Update the kfunc and the corresponding compatibility helper to implement
> reference acquisition and prevent potential RCU warnings.

I think KF_RCU likely fits better for peeking kernel data structures than
having to acquire/release them. Can you post the full backtrace? Is it being
called from a sleepable bpf prog? Or is it that we just need to expand the
rcu check scope to cover regular rcu, bh and sched? And, everything aside,
if KF_RCU, should we be tripping on rcu_dereference() in the first place?

Thanks.

-- 
tejun