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Date: Mon, 23 Mar 2015 14:39:44 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: mingo@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        yuyang.du@intel.com, preeti@linux.vnet.ibm.com, mturquette@linaro.org,
        nico@linaro.org, rjw@rjwysocki.net, juri.lelli@arm.com,
        linux-kernel@vger.kernel.org, Russell King <linux@arm.linux.org.uk>
Subject: Re: [RFCv3 PATCH 14/48] arm: Frequency invariant scheduler
 load-tracking support
Message-ID: <20150323133944.GG23123@twins.programming.kicks-ass.net>
References: <1423074685-6336-1-git-send-email-morten.rasmussen@arm.com>
 <1423074685-6336-15-git-send-email-morten.rasmussen@arm.com>
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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?