Re: [RFC][PATCH 2/3] sched: Add support to pick functions to take rf

[Peter Zijlstra]

On Mon, Oct 13, 2025 at 01:04:49PM +0200, Peter Zijlstra wrote:

> Bah; so yeah, this new behaviour is better for indeed always calling
> newidle when it is needed, but you're also right that in case of ext
> this might not be ideal.
> 
> So I have a pile of newidle hacks here:
> 
>   https://lkml.kernel.org/r/20251010170937.GG4067720@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> 
> and while I don't particularly like NI_SPARE (the has_spare_tasks thing
> is fickle); the idea seems to have some merit for this situation --
> where we know we'll not be having fair tasks at all.
> 
> I mean, we can always do something like this to sched_balance_newidle():
> 
> 	if (scx_switched_all())
> 		return 0;
> 
> Not pretty, but should do the job.

Oh, never mind, none of this is needed.

__pick_next_task()

	if (scx_enabled())
	  goto restart;

	...
restart:
	for_each_active_class(class) {
		...
	}


And then we have next_active_class() skip fair_sched_class entirely when
scx_switch_all().

So in the common ext case, we'll not hit pick_next_task_fair() at all.