Re: [PATCH sched_ext/for-7.1] tools/sched_ext: Kick idle CPU for pinned tasks in scx_qmap

[Cheng-Yang Chou]

Hi Tejun,

On Sat, Apr 11, 2026 at 01:33:56AM -1000, Tejun Heo wrote:
> scx_qmap uses global BPF queue maps for task dispatch. A task pinned to a
> single CPU can only be dispatched by its home CPU's ops.dispatch(), but an
> idle CPU won't call ops.dispatch() on its own. This leaves per-CPU kthreads
> like ksoftirqd stranded, causing NOHZ tick-stop warnings from pending
> softirqs.
> 
> Kick the target CPU with SCX_KICK_IDLE when enqueueing a pinned task.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> ---
>  tools/sched_ext/scx_qmap.bpf.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
> index f3587fb709c9..09d1624fb869 100644
> --- a/tools/sched_ext/scx_qmap.bpf.c
> +++ b/tools/sched_ext/scx_qmap.bpf.c
> @@ -314,6 +314,14 @@ void BPF_STRUCT_OPS(qmap_enqueue, struct task_struct *p, u64 enq_flags)
>  		__sync_fetch_and_add(&nr_highpri_queued, 1);
>  	}
>  	__sync_fetch_and_add(&nr_enqueued, 1);
> +
> +	/*
> +	 * Kick idle target CPU for pinned tasks. Without this, the CPU can
> +	 * idle while ksoftirqd is pending in the BPF queue, triggering NOHZ
> +	 * tick-stop warnings.
> +	 */
> +	if (p->nr_cpus_allowed == 1)
> +		scx_bpf_kick_cpu(scx_bpf_task_cpu(p), SCX_KICK_IDLE);
>  }
> 
>  /*
> --
> 2.53.0

Looks good to me! The same issue exists in scx_userland where pinned
tasks can be dispatched to SCX_DSQ_GLOBAL without kicking the idle
target CPU. I'll follow a patch to add the same fix there!

Reviewed-by: Cheng-Yang Chou <yphbchou0911@xxxxxxxxx>

-- 
Thanks,
Cheng-Yang