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

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Date:	Mon, 20 Jul 2015 16:38:26 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Sai Gurrappadi <sgurrappadi@...dia.com>
Cc:	peterz@...radead.org, mingo@...hat.com, 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

On Thu, Jul 16, 2015 at 05:10:52PM -0700, Sai Gurrappadi wrote:
> 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.

Right, this heuristic isn't as good as I had hoped for.
task_fits_capacity() is using capacity_of() to check if we have
available capacity after subtracting RT/IRQ activity which should be
right but I only check the first cpu. So I might discard a group due to
RT/IRQ activity on cpu the first cpu while one the sibling cpus could be
fine. Then going for lowest capacity_of() means preferring group with
most RT/IRQ activity that still has enough capacity to fit the task.

Using capacity_orig_of() we would ignore RT/IRQ activity but is likely
to be better as we can try to avoid RQ/IRQ activity later. I will use
capacity_orig_of() here instead. Thanks.

> > +
> > +	/* 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?

Yes, we should skip cpus with too much RT/IRQ activity here. Thanks.

Morten
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