linux-kernel - Re: [PATCHSET sched_ext/for-6.15] sched_ext: Enhance built-in idle selection with preferred CPUs

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Message-ID: <Z8nqpyEQmmff9E8X@slm.duckdns.org>
Date: Thu, 6 Mar 2025 08:34:15 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET sched_ext/for-6.15] sched_ext: Enhance built-in idle
 selection with preferred CPUs

Hello,

On Thu, Mar 06, 2025 at 07:18:03PM +0100, Andrea Righi wrote:
> To implement this, introduce a new helper kfunc scx_bpf_select_cpu_pref()
> that allows to specify a cpumask of preferred CPUs:
> 
> s32 scx_bpf_select_cpu_pref(struct task_struct *p,
> 			    const struct cpumask *preferred_cpus,
> 			    s32 prev_cpu, u64 wake_flags, u64 flags);
> 
> Moreover, introduce the new idle flag %SCX_PICK_IDLE_IN_PREF that can be
> used to enforce selection strictly within the preferred domain.

Would something like scx_bpf_select_cpu_and() work which is only allowed
pick in the intersection (ie. always SCX_PICK_IDLE_IN_PREF). I'm not sure
how much more beneficial a built-in two-level mechanism is especially given
that it wouldn't be too uncommon to need multi-level pick - e.g. within l3
then within numa node and so on.

Thanks.

-- 
tejun