linux-kernel - Re: [PATCH v5 2/3] sched/topology: Rework CPU capacity asymmetry detection

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Date:   Thu, 27 May 2021 09:03:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Beata Michalska <beata.michalska@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org, corbet@....net,
        rdunlap@...radead.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 2/3] sched/topology: Rework CPU capacity asymmetry
 detection

On Wed, May 26, 2021 at 11:52:25AM +0200, Dietmar Eggemann wrote:

> For me asym_cpu_capacity_classify() is pretty hard to digest ;-) But I
> wasn't able to break it. It also performs correctly on (non-existing SMT)
> layer (with sd span eq. single CPU).

This is the simplest form I could come up with this morning:

static inline int
asym_cpu_capacity_classify(struct sched_domain *sd,
                          const struct cpumask *cpu_map)
{
	struct asym_cap_data *entry;
	int i = 0, n = 0;

	list_for_each_entry(entry, &asym_cap_list, link) {
		if (cpumask_intersects(sched_domain_span(sd), entry->cpu_mask))
			i++;
		else
			n++;
	}

	if (WARN_ON_ONCE(!i) || i == 1) /* no asymmetry */
		return 0;

	if (n) /* partial asymmetry */
		return SD_ASYM_CPUCAPACITY;

	/* full asymmetry */
	return SD_ASYM_CPUCAPACITY | SD_ASYM_CPUCAPACITY_FULL;
}


The early termination and everything was cute; but this isn't
performance critical code and clarity is paramount.