linux-kernel - Re: [RFC v2 0/3][TESTS] LAB: Support for Legacy Application Booster governor

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Message-ID: <519F2D89.5030602@linaro.org>
Date:	Fri, 24 May 2013 11:06:17 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	Lukasz Majewski <l.majewski@...sung.com>,
	Jonghwa Lee <jonghwa3.lee@...sung.com>,
	"Rafael J. Wysocky" <rjw@...k.pl>, linux-kernel@...r.kernel.org,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	Vicent Guittot <vincent.guittot@...aro.org>,
	MyungJoo Ham <myungjoo.ham@...sung.com>,
	Lukasz Majewski <l.majewski@...ess.pl>
Subject: Re: [RFC v2 0/3][TESTS] LAB: Support for Legacy Application Booster
 governor - tests results

On 05/24/2013 10:51 AM, Viresh Kumar wrote:
> On 24 May 2013 14:00, Lukasz Majewski <l.majewski@...sung.com> wrote:
>> The overclock frequency (1.5 GHz) is possible to set as an ordinary,
>> available frequency (policy->max) for ondemand.
>>
>> Unfortunately with our load patterns, this frequency rapidly increases
>> internal chip temperature (chip goes out of available power/thermal
>> dissipation range), and consumes extra power when not needed.
>>
>> The core idea with overclock is to increase ("boost") the frequency
>> when conditions allow to do it (for example load is affined to a single
>> core, other are idle). Then we will not exceed power/thermal budget, but
>> increase performance (and even save power).
>>
>>
>> Overclocking is efficiently utilized by LAB, which relies on a number of
>> idle cpus. Thus, we can easily asses if we can enable it.
>>
>> I also foresee potential use of overclocking, when scheduler will take a
>> major role of power saver for mobile (ARM) linux. Since it will try to
>> pack as much tasks as possible to a single core - it will need a
>> framework/API to "boost" their execution.
> 
> Okay.. so its exactly what I thought the reason would be.
> 
> What I would have done if I was in your place is:
> 
> Add following sysfs tunables to ondemand governor:
> 
> - overdrive_freq: We will go over this frequency only when
> number of busy cores is <= overdrive_cores..
> For your case it will be 1.4 GHz
> 
> - overdrive_cores: We will enable overdrive frequencies only if no. of
> busy cores is <= overdrive_cores. Zero by default (So, that this feature
> is disabled by default) and 1 for your case.
> 
> And your driver will include all the available frequencies in the freq
> table.
> 
> I hope this will be the most generic solution to your problem..

I agree with Viresh, a new governor is not necessary here for that.

There is the /sys/devices/system/cpufreq/boost option existing for x86
platform, why do not reuse it ? It is supposed to do exactly what you
want to achieve.

IMO, the logic of boosting one core when the other are idle should be in
the driver itself and certainly not setup by the user, except if we
consider acceptable the user can burn its board ... :)

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