linux-kernel - Re: [RFCv3 PATCH 14/48] arm: Frequency invariant scheduler load-tracking support

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Message-ID: <20150323133944.GG23123@twins.programming.kicks-ass.net>
Date:	Mon, 23 Mar 2015 14:39:44 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	mingo@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, yuyang.du@...el.com,
	preeti@...ux.vnet.ibm.com, mturquette@...aro.org, nico@...aro.org,
	rjw@...ysocki.net, juri.lelli@....com,
	linux-kernel@...r.kernel.org, Russell King <linux@....linux.org.uk>
Subject: Re: [RFCv3 PATCH 14/48] arm: Frequency invariant scheduler
 load-tracking support

On Wed, Feb 04, 2015 at 06:30:51PM +0000, Morten Rasmussen wrote:
> +/* cpufreq callback function setting current cpu frequency */
> +void arch_scale_set_curr_freq(int cpu, unsigned long freq)
> +{
> +	atomic_long_set(&per_cpu(cpu_curr_freq, cpu), freq);
> +}
> +
> +/* cpufreq callback function setting max cpu frequency */
> +void arch_scale_set_max_freq(int cpu, unsigned long freq)
> +{
> +	atomic_long_set(&per_cpu(cpu_max_freq, cpu), freq);
> +}
> +
> +unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu)
> +{
> +	unsigned long curr = atomic_long_read(&per_cpu(cpu_curr_freq, cpu));
> +	unsigned long max = atomic_long_read(&per_cpu(cpu_max_freq, cpu));
> +
> +	if (!curr || !max)
> +		return SCHED_CAPACITY_SCALE;
> +
> +	return (curr * SCHED_CAPACITY_SCALE) / max;
> +}

so I've no idea how many cycles an (integer) division takes on ARM; but
doesn't it make sense to do this division (once) in
arch_scale_set_curr_freq() instead of every time we need the result?
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