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: <Z8nvam-WarNqdLw9@gpd3>
Date: Thu, 6 Mar 2025 19:54:34 +0100
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
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

Hi Tejun,

On Thu, Mar 06, 2025 at 08:34:15AM -1000, Tejun Heo wrote:
> 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.

Just to make sure I understand, you mean provide two separate kfuncs:
scx_bpf_select_cpu_and() and scx_bpf_select_cpu_pref(), instead of
introducing the flag?

Thanks,
-Andrea