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Date: Wed, 18 Apr 2018 17:23:16 +0800
From: Leo Yan <leo.yan@linaro.org>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
	Quentin Perret <quentin.perret@arm.com>,
	Thara Gopinath <thara.gopinath@linaro.org>,
	linux-pm@vger.kernel.org,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Chris Redpath <chris.redpath@arm.com>,
	Patrick Bellasi <patrick.bellasi@arm.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Todd Kjos <tkjos@google.com>, Joel Fernandes <joelaf@google.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Steve Muckle <smuckle@google.com>,
	Eduardo Valentin <edubezval@gmail.com>
Subject: Re: [RFC PATCH v2 4/6] sched/fair: Introduce an energy estimation
 helper function
Message-ID: <20180418092316.GB15682@leoy-ThinkPad-X240s>
References: <20180406153607.17815-1-dietmar.eggemann@arm.com>
 <20180406153607.17815-5-dietmar.eggemann@arm.com>
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On Fri, Apr 06, 2018 at 04:36:05PM +0100, Dietmar Eggemann wrote:

[...]

> +/*
> + * Estimates the system level energy assuming that p wakes-up on dst_cpu.
> + *
> + * compute_energy() is safe to call only if an energy model is available for
> + * the platform, which is when sched_energy_enabled() is true.
> + */
> +static unsigned long compute_energy(struct task_struct *p, int dst_cpu)
> +{
> +	unsigned long util, max_util, sum_util;
> +	struct capacity_state *cs;
> +	unsigned long energy = 0;
> +	struct freq_domain *fd;
> +	int cpu;
> +
> +	for_each_freq_domain(fd) {
> +		max_util = sum_util = 0;
> +		for_each_cpu_and(cpu, freq_domain_span(fd), cpu_online_mask) {
> +			util = cpu_util_next(cpu, p, dst_cpu);
> +			util += cpu_util_dl(cpu_rq(cpu));
> +			max_util = max(util, max_util);
> +			sum_util += util;
> +		}
> +
> +		/*
> +		 * Here we assume that the capacity states of CPUs belonging to
> +		 * the same frequency domains are shared. Hence, we look at the
> +		 * capacity state of the first CPU and re-use it for all.
> +		 */
> +		cpu = cpumask_first(freq_domain_span(fd));
> +		cs = find_cap_state(cpu, max_util);
> +		energy += cs->power * sum_util / cs->cap;

I am a bit worry about the resolution issue, especially when the
capacity value is a quite high value and sum_util is a minor value.

> +	}
> +
> +	return energy;
> +}