linux-kernel - Re: [PATCH v4 5/7] cpufreq: Calculate number of busy CPUs

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Date:	Thu, 4 Jul 2013 10:36:53 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Lukasz Majewski <l.majewski@...sung.com>
Cc:	"Rafael J. Wysocky" <rjw@...k.pl>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Jonghwa Lee <jonghwa3.lee@...sung.com>,
	Myungjoo Ham <myungjoo.ham@...sung.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Lukasz Majewski <l.majewski@...ess.pl>,
	Andre Przywara <andre.przywara@...aro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Zhang Rui <rui.zhang@...el.com>,
	Eduardo Valentin <eduardo.valentin@...com>
Subject: Re: [PATCH v4 5/7] cpufreq: Calculate number of busy CPUs

Hi Lukasz,

Sorry for being late. Actually I didn't had an answer to your mail
and wanted to go through it with some fresh mind. This is my
first mail this morning, lets see if I can bring something good
into the discussion.

On 1 July 2013 13:45, Lukasz Majewski <l.majewski@...sung.com> wrote:
> Does anybody have any idea here? As written above, thermal is suitable
> to disable boost.

See, one thing is very clear. User space applications aren't responsible
for enabling boost again and again. There has to be a internal mechanism
inside kernel for that.

> I'd like to bring those three options under discussion:
>
> 1. boost attr is always exported -> do not enable boost automatically
>    when disabled by thermal (as it was proposed at v4).

So, that's a problem. I see one more solution to that.
- Create another Macro in cpufreq.c which would contain the time
after which we will autoenable boost.
- So, suppose thermal disabled it due to high temperature (Lets not
change value of sysfs variable boost_enable, but create another
variable like: skip_boost: which means skip boost temporarily).
- Thermal would enable this variable skip_boost.
- Then we will continue to get requests for next frequency and will
check eveytime if we have exceeded time for autoenabling boost.
- If yes, then we disable this variable and start boosting again..
- Then thermal can disable it again later.

This variable (time for autoenable) looks to be more platform
dependent for now, but lets don't make it like that unless somebody
needs it.

> 2. boost attr is always exported -> find a way to enable boost after
>    emergency disablement when thermal detects overheating (newest
>    proposition).

My solution above probably.

> 3. boost attr only exported at x86 (when supported)
>    boost attr NOT exported via sysfs for SW controlled boost (e.g.
>    Exynos ARM).
>
>    Then we only enable/disable boost at kernel and don't need to take
>    care of the user space interaction. This scenario is my use case. I
>    hadn't planned to expose boost to userspace and use it with LAB as a
>    kernel API.

Userspace must have control of this feature after kernel is built. That kernel
image may run for ever without changing in a product.

@Rafael: How crazy do you think my solution is? :)
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