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From: Dietmar Eggemann <dietmar.eggemann@arm.com>
Subject: Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler
 utilization data
To: Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>
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Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Linux PM list <linux-pm@vger.kernel.org>,
        Juri Lelli <juri.lelli@arm.com>,
        Steve Muckle <steve.muckle@linaro.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Michael Turquette <mturquette@baylibre.com>
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Date: Thu, 3 Mar 2016 17:28:55 +0000
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On 03/03/16 16:28, Peter Zijlstra wrote:
> On Thu, Mar 03, 2016 at 04:38:17PM +0100, Peter Zijlstra wrote:
>> On Thu, Mar 03, 2016 at 03:01:15PM +0100, Vincent Guittot wrote:
>>>> In case a more formal derivation of this formula is needed, it is
>>>> based on the following 3 assumptions:
>>>>
>>>> (1) Performance is a linear function of frequency.
>>>> (2) Required performance is a linear function of the utilization ratio
>>>> x = util/max as provided by the scheduler (0 <= x <= 1).
>>>
>>> Just to mention that the utilization that you are using, varies with
>>> the frequency which add another variable in your equation
>>
>> Right, x86 hasn't implemented arch_scale_freq_capacity(), so the
>> utilization values we use are all over the map. If we lower freq, the
>> util will go up, which would result in us bumping the freq again, etc..
> 
> Something like the completely untested below should maybe work.
> 
> Rafael?
> 

[...]

> +void arch_scale_freq_tick(void)
> +{
> +	u64 aperf, mperf;
> +	u64 acnt, mcnt;
> +
> +	if (!static_cpu_has(X86_FEATURE_APERFMPERF))
> +		return;
> +
> +	aperf = rdmsrl(MSR_IA32_APERF);
> +	mperf = rdmsrl(MSR_IA32_APERF);
> +
> +	acnt = aperf - this_cpu_read(arch_prev_aperf);
> +	mcnt = mperf - this_cpu_read(arch_prev_mperf);
> +
> +	this_cpu_write(arch_prev_aperf, aperf);
> +	this_cpu_write(arch_prev_mperf, mperf);
> +
> +	this_cpu_write(arch_cpu_freq, div64_u64(acnt * SCHED_CAPACITY_SCALE, mcnt));

Wasn't there the problem that this ratio goes to zero if the cpu is idle
in the old power estimation approach on x86?

[...]