linux-kernel - Re: [PATCH 6/6] sched_ext: idle: Introduce node-aware idle cpu kfunc helpers

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Message-ID: <Z6aLvYaYlQ3KRZQM@slm.duckdns.org>
Date: Fri, 7 Feb 2025 12:39:57 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>, Ian May <ianm@...dia.com>,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] sched_ext: idle: Introduce node-aware idle cpu kfunc
 helpers

Hello,

On Fri, Feb 07, 2025 at 09:40:53PM +0100, Andrea Righi wrote:
> +/**
> + * scx_bpf_cpu_to_node - Return the NUMA node the given @cpu belongs to
> + */
> +__bpf_kfunc int scx_bpf_cpu_to_node(s32 cpu)

Maybe scx_bpf_cpu_node() to be in line with scx_bpf_task_cpu/cgroup()?

> +{
> +#ifdef CONFIG_NUMA
> +	if (cpu < 0 || cpu >= nr_cpu_ids)
> +		return -EINVAL;

Use ops_cpu_valid()? Otherwise, we can end up calling cpu_to_node() with an
impossible CPU. Also, I don't think CPU -> node mapping function should be
able to return an error value. It should just trigger ops error.

> +
> +	return idle_cpu_to_node(cpu);

This is contingent on scx_builtin_idle_per_node, right? It's confusing for
CPU -> node mapping function to return NUMA_NO_NODE depending on an ops
flag. Shouldn't this be a generic mapping function?

> index 50e1499ae0935..caa1a80f9a60c 100644
> --- a/tools/sched_ext/include/scx/compat.bpf.h
> +++ b/tools/sched_ext/include/scx/compat.bpf.h
> @@ -130,6 +130,25 @@ bool scx_bpf_dispatch_vtime_from_dsq___compat(struct bpf_iter_scx_dsq *it__iter,
>  	 scx_bpf_now() :							\
>  	 bpf_ktime_get_ns())
>  
> +#define __COMPAT_scx_bpf_cpu_to_node(cpu)					\
> +	(bpf_ksym_exists(scx_bpf_cpu_to_node) ?					\
> +	 scx_bpf_cpu_to_node(cpu) : 0)
> +
> +#define __COMPAT_scx_bpf_get_idle_cpumask_node(node)				\
> +	(bpf_ksym_exists(scx_bpf_get_idle_cpumask_node) ?			\
> +	 scx_bpf_get_idle_cpumask_node(node) :					\
> +	 scx_bpf_get_idle_cpumask())						\
> +
> +#define __COMPAT_scx_bpf_get_idle_smtmask_node(node)				\
> +	(bpf_ksym_exists(scx_bpf_get_idle_smtmask_node) ?			\
> +	 scx_bpf_get_idle_smtmask_node(node) :					\
> +	 scx_bpf_get_idle_smtmask())
> +
> +#define __COMPAT_scx_bpf_pick_idle_cpu_node(cpus_allowed, node, flags)		\
> +	(bpf_ksym_exists(scx_bpf_pick_idle_cpu_node) ?				\
> +	 scx_bpf_pick_idle_cpu_node(cpus_allowed, node, flags) :		\
> +	 scx_bpf_pick_idle_cpu(cpus_allowed, flags))

Can you please document when these compat macros can be dropped? Also,
shouldn't it also provide a compat macro for the new ops flag using
__COMPAT_ENUM_OR_ZERO()? Otherwise, trying to load new binary using the new
flag on an older kernel will fail, right?

Thanks.

-- 
tejun