linux-kernel - Re: [PATCH v3 1/3] sched/fair: Add asymmetric CPU capacity wakeup scan

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Date:   Wed, 29 Jan 2020 12:04:55 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Cc:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        morten.rasmussen@....com, qperret@...gle.com,
        adharmap@...eaurora.org
Subject: Re: [PATCH v3 1/3] sched/fair: Add asymmetric CPU capacity wakeup
 scan

On 26/01/2020 21:09, Valentin Schneider wrote:

[...]

> +static int select_idle_capacity(struct task_struct *p, int target)
> +{
> +	unsigned long best_cap = 0;
> +	struct sched_domain *sd;
> +	struct cpumask *cpus;
> +	int best_cpu = -1;
> +	struct rq *rq;
> +	int cpu;
> +
> +	if (!static_branch_unlikely(&sched_asym_cpucapacity))
> +		return -1;
> +
> +	sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, target));
> +	if (!sd)
> +		return -1;
> +
> +	sync_entity_load_avg(&p->se);
> +
> +	cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
> +	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +
> +	for_each_cpu_wrap(cpu, cpus, target) {
> +		rq = cpu_rq(cpu);
> +
> +		if (!available_idle_cpu(cpu))
> +			continue;
> +		if (task_fits_capacity(p, rq->cpu_capacity))
> +			return cpu;
> +
> +		/*
> +		 * It would be silly to keep looping when we've found a CPU
> +		 * of highest available capacity. Just check that it's not been
> +		 * too pressured lately.
> +		 */
> +		if (rq->cpu_capacity_orig == READ_ONCE(rq->rd->max_cpu_capacity) &&

There is a similar check in check_misfit_status(). Common helper function?

> +		    !check_cpu_capacity(rq, sd))
> +			return cpu;

I wonder how this special treatment of a big CPU behaves in (LITTLE,
medium, big) system like Pixel4 (Snapdragon 855):

 flame:/ $ cat /sys/devices/system/cpu/cpu*/cpu_capacity

261
261
261
261
871
871
871
1024

Or on legacy systems where the sd->imbalance_pct is 25% instead of 17%?