linux-kernel - Re: cpuidle future and improvements

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Message-ID: <CAORVsuWLLp1v4uQeKCqtSyZNZ8Mj=xA-1ZOfk+OAkHvHMEcwUg@mail.gmail.com>
Date:	Mon, 18 Jun 2012 15:06:12 +0200
From:	Jean Pihet <jean.pihet@...oldbits.com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	Peter De Schrijver <pdeschrijver@...dia.com>,
	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-pm@...ts.linux-foundation.org" 
	<linux-pm@...ts.linux-foundation.org>,
	Lists Linaro-dev <linaro-dev@...ts.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	"lenb@...nel.org" <lenb@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Colin Cross <ccross@...roid.com>, Rob Lee <rob.lee@...aro.org>,
	"rjw@...k.pl" <rjw@...k.pl>, Kevin Hilman <khilman@...com>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>
Subject: Re: cpuidle future and improvements

Hi Daniel,

On Mon, Jun 18, 2012 at 2:55 PM, Daniel Lezcano
<daniel.lezcano@...aro.org> wrote:
> On 06/18/2012 02:53 PM, Peter De Schrijver wrote:
>> On Mon, Jun 18, 2012 at 02:35:42PM +0200, Daniel Lezcano wrote:
>>> On 06/18/2012 01:54 PM, Deepthi Dharwar wrote:
>>>> On 06/18/2012 02:10 PM, Daniel Lezcano wrote:
>>>>
>>>>>
>>>>> Dear all,
>>>>>
>>>>> A few weeks ago, Peter De Schrijver proposed a patch [1] to allow per
>>>>> cpu latencies. We had a discussion about this patchset because it
>>>>> reverse the modifications Deepthi did some months ago [2] and we may
>>>>> want to provide a different implementation.
>>>>>
>>>>> The Linaro Connect [3] event bring us the opportunity to meet people
>>>>> involved in the power management and the cpuidle area for different SoC.
>>>>>
>>>>> With the Tegra3 and big.LITTLE architecture, making per cpu latencies
>>>>> for cpuidle is vital.
>>>>>
>>>>> Also, the SoC vendors would like to have the ability to tune their cpu
>>>>> latencies through the device tree.
>>>>>
>>>>> We agreed in the following steps:
>>>>>
>>>>> 1. factor out / cleanup the cpuidle code as much as possible
>>>>> 2. better sharing of code amongst SoC idle drivers by moving common bits
>>>>> to core code
>>>>> 3. make the cpuidle_state structure contain only data
>>>>> 4. add a API to register latencies per cpu
That makes sense, especially if you can refactor _and_ add new
functionality at the same time.

>>>> On huge systems especially  servers, doing a cpuidle registration on a
>>>> per-cpu basis creates a big overhead.
>>>> So global registration was introduced in the first place.
>>>>
>>>> Why not have it as a configurable option or so ?
>>>> Architectures having uniform cpuidle state parameters can continue to
>>>> use global registration, else have an api to register latencies per cpu
>>>> as proposed. We can definitely work to see the best way to implement it.
>>>
>>> Absolutely, this is one reason I think adding a function:
>>>
>>> cpuidle_register_latencies(int cpu, struct cpuidle_latencies);
>>>
>>> makes sense if it is used only for cpus with different latencies.
>>> The other architecture will be kept untouched.
Do you mean by keeping the parameters in the cpuidle_driver struct and
not calling the new API? That looks great.

>>>
>>> IMHO, before adding more functionalities to cpuidle, we should cleanup
>>> and consolidate the code. For example, there is a dependency between
>>> acpi_idle and intel_idle which can be resolved with the notifiers, or
>>> there is intel specific code in cpuidle.c and cpuidle.h, cpu_relax is
>>> also introduced to cpuidle which is related to x86 not the cpuidle core,
>>> etc ...
>>>
>>> Cleanup the code will help to move the different bits from the arch
>>> specific code to the core code and reduce the impact of the core's
>>> modifications. That should let a common pattern to emerge and will
>>> facilitate the modifications in the future (per cpu latencies is one of
>>> them).
>>>
>>> That will be a lot of changes and this is why I proposed to put in place
>>> a cpuidle-next tree in order to consolidate all the cpuidle
>>> modifications people is willing to see upstream and provide better testing.
Nice! The new tree needs to be as close as possible to mainline
though. Do you have plans for that?
Do not hesitate to ask for help on OMAPs!

Regards,
Jean

>>
>> Sounds like a good idea. Do you have something like that already?
>
> Yes but I need to cleanup the tree before.
>
> http://git.linaro.org/gitweb?p=people/dlezcano/linux-next.git;a=summary
>
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