linux-kernel - Re: [PATCH 1/6] sched_ext: idle: Extend topology optimizations to all tasks

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Message-ID: <Z9hoa5iPpDEOnXKt@slm.duckdns.org>
Date: Mon, 17 Mar 2025 08:22:35 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	Joel Fernandes <joelagnelf@...dia.com>, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] sched_ext: idle: Extend topology optimizations to
 all tasks

Hello,

On Mon, Mar 17, 2025 at 06:53:24PM +0100, Andrea Righi wrote:
> +/*
> + * Return the subset of @cpus that task @p can use or NULL if none of the
> + * CPUs in the @cpus cpumask can be used.
> + */
> +static const struct cpumask *task_cpumask(const struct task_struct *p, const struct cpumask *cpus,
> +					  struct cpumask *local_cpus)

task_cpus_allowed_and()? It also would help to add comment explaining the
parameters as the function is a bit unusual.

> +{
> +	/*
> +	 * If the task is allowed to run on all CPUs, simply use the
> +	 * architecture's cpumask directly. Otherwise, compute the
> +	 * intersection of the architecture's cpumask and the task's
> +	 * allowed cpumask.
> +	 */
> +	if (!cpus || p->nr_cpus_allowed >= num_possible_cpus() ||
> +	    cpumask_subset(cpus, p->cpus_ptr))
> +		return cpus;
> +
> +	if (!cpumask_equal(cpus, p->cpus_ptr) &&

Hmm... isn't this covered by the preceding cpumask_subset() test? Here, cpus
is not a subset of p->cpus_ptr, so how can it be the same as p->cpus_ptr?

> +	    cpumask_and(local_cpus, cpus, p->cpus_ptr))
> +		return local_cpus;
> +
> +	return NULL;

and return values need some explanation too.

Thanks.

-- 
tejun