linux-kernel - Re: [RFCv5 PATCH 32/46] sched: Energy-aware wake-up task placement

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Date:	Thu, 16 Jul 2015 17:10:52 -0700
From:	Sai Gurrappadi <sgurrappadi@...dia.com>
To:	Morten Rasmussen <morten.rasmussen@....com>,
	<peterz@...radead.org>, <mingo@...hat.com>
CC:	<vincent.guittot@...aro.org>, <daniel.lezcano@...aro.org>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	<yuyang.du@...el.com>, <mturquette@...libre.com>,
	<rjw@...ysocki.net>, Juri Lelli <Juri.Lelli@....com>,
	<pang.xunlei@....com.cn>, <linux-kernel@...r.kernel.org>,
	<linux-pm@...r.kernel.org>, <pboonstoppel@...dia.com>
Subject: Re: [RFCv5 PATCH 32/46] sched: Energy-aware wake-up task placement

Hi Morten,

On 07/07/2015 11:24 AM, Morten Rasmussen wrote:
> ---

> +static int energy_aware_wake_cpu(struct task_struct *p, int target)
> +{
> +	struct sched_domain *sd;
> +	struct sched_group *sg, *sg_target;
> +	int target_max_cap = INT_MAX;
> +	int target_cpu = task_cpu(p);
> +	int i;
> +
> +	sd = rcu_dereference(per_cpu(sd_ea, task_cpu(p)));
> +
> +	if (!sd)
> +		return target;
> +
> +	sg = sd->groups;
> +	sg_target = sg;
> +
> +	/*
> +	 * Find group with sufficient capacity. We only get here if no cpu is
> +	 * overutilized. We may end up overutilizing a cpu by adding the task,
> +	 * but that should not be any worse than select_idle_sibling().
> +	 * load_balance() should sort it out later as we get above the tipping
> +	 * point.
> +	 */
> +	do {
> +		/* Assuming all cpus are the same in group */
> +		int max_cap_cpu = group_first_cpu(sg);
> +
> +		/*
> +		 * Assume smaller max capacity means more energy-efficient.
> +		 * Ideally we should query the energy model for the right
> +		 * answer but it easily ends up in an exhaustive search.
> +		 */
> +		if (capacity_of(max_cap_cpu) < target_max_cap &&
> +		    task_fits_capacity(p, max_cap_cpu)) {
> +			sg_target = sg;
> +			target_max_cap = capacity_of(max_cap_cpu);
> +		}
> +	} while (sg = sg->next, sg != sd->groups);

Should be capacity_orig_of(max_cap_cpu) right? Might select a suboptimal
sg_target if max_cap_cpu has a significant amount of IRQ/RT activity.

> +
> +	/* Find cpu with sufficient capacity */
> +	for_each_cpu_and(i, tsk_cpus_allowed(p), sched_group_cpus(sg_target)) {
> +		/*
> +		 * p's blocked utilization is still accounted for on prev_cpu
> +		 * so prev_cpu will receive a negative bias due the double
> +		 * accouting. However, the blocked utilization may be zero.
> +		 */
> +		int new_usage = get_cpu_usage(i) + task_utilization(p);
> +
> +		if (new_usage >	capacity_orig_of(i))
> +			continue;

Is this supposed to be capacity_of(i) instead?

> +
> +		if (new_usage <	capacity_curr_of(i)) {
> +			target_cpu = i;
> +			if (cpu_rq(i)->nr_running)
> +				break;
> +		}
> +
> +		/* cpu has capacity at higher OPP, keep it as fallback */
> +		if (target_cpu == task_cpu(p))
> +			target_cpu = i;
> +	}
> +
> +	if (target_cpu != task_cpu(p)) {
> +		struct energy_env eenv = {
> +			.usage_delta	= task_utilization(p),
> +			.src_cpu	= task_cpu(p),
> +			.dst_cpu	= target_cpu,
> +		};
> +
> +		/* Not enough spare capacity on previous cpu */
> +		if (cpu_overutilized(task_cpu(p)))
> +			return target_cpu;
> +
> +		if (energy_diff(&eenv) >= 0)
> +			return task_cpu(p);
> +	}
> +
> +	return target_cpu;
> +}
> +

-Sai
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