linux-kernel - Re: [RFC PATCH 3/3] idle: store the idle state index in the struct rq

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Message-ID: <52EBAF37.90900@arm.com>
Date:	Fri, 31 Jan 2014 14:12:07 +0000
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Len Brown <len.brown@...el.com>
CC:	Preeti Murthy <preeti.lkml@...il.com>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>
Subject: Re: [RFC PATCH 3/3] idle: store the idle state index in the struct
 rq

On 31/01/14 14:04, Daniel Lezcano wrote:
> On 01/31/2014 10:39 AM, Preeti U Murthy wrote:
>> Hi Peter,
>>
>> On 01/31/2014 02:32 PM, Peter Zijlstra wrote:
>>> On Fri, Jan 31, 2014 at 02:15:47PM +0530, Preeti Murthy wrote:
>>>>>
>>>>> If the driver does its own random mapping that will break the governor
>>>>> logic. So yes, the states are ordered, the higher the index is, the more you
>>>>> save power and the higher the exit latency is.
>>>>
>>>> The above point holds true for only the ladder governor which sees the idle
>>>> states indexed in the increasing order of target_residency/exit_latency.
>>>>
>>>> However this is not true as far as I can see in the menu governor. It
>>>> acknowledges the dynamic ordering of idle states as can be seen in the
>>>> menu_select() function in the menu governor, where the idle state for the
>>>> CPU gets chosen.  You will notice that, even if it is found that the predicted
>>>> idle time of the CPU is smaller than the target residency of an idle state,
>>>> the governor continues to search for suitable idle states in the higher indexed
>>>> states although it should have halted if the idle states' were ordered according
>>>> to their target residency.. The same holds for exit_latency.
>>>>
>>>> Hence I think this patch would make sense only with additional information
>>>> like exit_latency or target_residency is present for the scheduler. The idle
>>>> state index alone will not be sufficient.
>>>
>>> Alternatively, can we enforce sanity on the cpuidle infrastructure to
>>> make the index naturally ordered? If not, please explain why :-)
>>
>> The commit id 71abbbf856a0e70 says that there are SOCs which could have
>> their target_residency and exit_latency values change at runtime. This
>> commit thus removed the ordering of the idle states according to their
>> target_residency/exit_latency. Adding Len and Arjan to the CC.
> 
> This commit is outdated, AFAICT.

Yes, this is also my impression. It's removed by

commit b25edc42bfb9602f0503474b2c94701d5536ce60
Author: Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>
Date:   Fri Oct 28 16:20:24 2011 +0530

    cpuidle: Remove CPUIDLE_FLAG_IGNORE and dev->prepare()

So far, I'm under the impression that target_residency/exit_latency is
static data and can be propagated towards the scheduler via topology
information.

-- Dietmar

> 
> Indeed, there are dynamic idle states. Some idle states are added or 
> removed when a laptop is going to battery or plugged in.
> 
> In ACPI, the power event leads the acpi cpuidle driver to disable the 
> cpuidle framework, get the idle states which are ordered, and re-enable 
> the cpuidle framework which in turn kicks all the cpus. So the index in 
> the struct rq should be always ok.
> 
> 
> 
> 


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