Re: [PATCH v2 03/11] sched_ext: Add an event, SELECT_CPU_FALLBACK

[Tejun Heo]

Hello,

On Sun, Jan 26, 2025 at 07:16:06PM +0900, Changwoo Min wrote:
...
>  struct scx_event_stats {
> +	/*
> +	 * If ops.select_cpu() returns a CPU which can't be used by the task,
> +	 * the core scheduler code silently picks a fallback CPU.
> +	 */
> +	u64		SELECT_CPU_FALLBACK;

As C has one global namespace, we're gonna have to prefix these. Hopefully,
something not too long. SCX_EV_?

> @@ -3663,6 +3668,10 @@ static int select_task_rq_scx(struct task_struct *p, int prev_cpu, int wake_flag
>  		cpu = SCX_CALL_OP_TASK_RET(SCX_KF_ENQUEUE | SCX_KF_SELECT_CPU,
>  					   select_cpu, p, prev_cpu, wake_flags);
>  		*ddsp_taskp = NULL;
> +
> +		if (unlikely(!is_cpu_allowed(p, cpu)))
> +			__scx_add_event(SELECT_CPU_FALLBACK, 1);

This is trying to guess what select_task_rq() is going to do and then count
that as an event, which doesn't seem great. Can't we just record the picked
CPU here into a field in p->scx and then compare that against cpu_of(rq) in
enqueue_task_scx()? That way, we can reliably count events where CPU
selection was modified by the fallback logic regardless of why or how that
happens.

Thanks.

-- 
tejun