linux-kernel - Re: [RFC PATCH 1/2] cpuidle: Add idle enter/exit time stamp for notifying current idle state.

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Date:	Tue, 02 Apr 2013 20:07:56 +0900
From:	jonghwa3.lee@...sung.com
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, cpufreq@...r.kernel.org,
	MyungJoo Ham <myungjoo.ham@...sung.com>,
	Lukasz Majewski <l.majewski@...sung.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Chanwoo Choi <cw00.choi@...sung.com>, sw0312.kim@...sung.com,
	m.szyprowski@...sung.com
Subject: Re: [RFC PATCH 1/2] cpuidle: Add idle enter/exit time stamp for
 notifying current idle state.

On 2013년 04월 02일 19:08, Daniel Lezcano wrote:

> On 04/02/2013 11:37 AM, jonghwa3.lee@...sung.com wrote:
>> On 2013년 04월 02일 16:34, Daniel Lezcano wrote:
>>
>>> On 04/02/2013 08:17 AM, jonghwa3.lee@...sung.com wrote:
>>>> On 2013년 04월 02일 14:00, Daniel Lezcano wrote:
>>>>
>>>>> On 04/01/2013 10:24 AM, Jonghwa Lee wrote:
>>>>>> This patch adds idle state time stamp to cpuidle device structure to
>>>>>> notify its current idle state. If last enter time is newer than last
>>>>>> exit time, then it means that the core is in idle now.
>>>>>>
>>>>>> Signed-off-by: Jonghwa Lee <jonghwa3.lee@...sung.com>
>>>>>> ---
>>>>>
>>>>> The patch description does not explain what problem you want to solve,
>>>>> how to solve it and the patch itself shows nothing.
>>>>>
>>>>> Could you elaborate ?
>>>>
>>>>
>>>> I'm sorry for lacking description. I supplement more.
>>>>
>>>> This patch does add time-stamp for idle enter/exit only nothing more.
>>>> The reason why I needed them is that I wanted to know current cpu idle
>>>> state. It is hard to know whether cpu is in idle or not now.
>>>
>>> Did you looked at:
>>>
>>> include/linux/sched.h:extern int idle_cpu(int cpu);
>>>
>>
>>
>> Yes, I did.
>>
>>> ?
>>>
>>>> When I check the cpuidle state usage, sometimes the information is wrong.
>>>> Because it is updated only when the cpu exits the idle state. So while the
>>>> cpu is idling, the cpuidle state usage holds past one. Therefore I put
>>>> the time-stamp for cpuidle enter/exit for checking current idling and
>>>> calculating idle state usage correctly.
>>>>
>>>> I just make this patch temporary for my cpufreq governor work. So, it just
>>>> use time-stamp for all idle state together. After RFC working, I have a plan
>>>> to update this patch to use timestamp for each idle state.
>>>
>>> I suggest you look at the enter_idle / exit_idle function and make your
>>> governor to subscribe to the IDLE_START/EXIT notifiers.
>>>
>>> arch/x86/kernel/process.c
>>>
>>> These are defined for the x86 architecture, maybe worth to add it to
>>> another architecture.
>>>
>>
>>
>> Thanks for your opinion.
>>
>> Actually, I work on ARM architecture and I knew that the attempt of applying
>> idle notifier was failed. You probably knew it, because the link you gave me
>> before is that attempt. (https://lkml.org/lkml/2012/2/7/504) :)
> 
> Yeah, now I recall this thread.
> 


Oh my, I thought you gave the link but you didn't. It was Viresh Kumar from
other patch of the patchset. Sorry.

>> Currently, there
>> is only notifying call which is for led in arch/arm/kernel/process.c and I think
>> it isn't for me to use. Anyway, Do you really think it is better way to use
>> notifier than my way? Because I think it is too heavy for me. On my board,
>> sometimes entering idle happened hundreds times during the 100ms. I don't want
>> to call notifier that much time. IMO, just moving local variable to per-cpu
>> variable for showing the enter/exit time looks better although it requires code
>> modification on cpudile side.  What do you think?
> 
> Sorry, but it is hard to figure out what you are trying to achieve with
> a single patch.
> 
> IIUC, you want to know how long the cpu is idle including the current
> state, right ? So you need to know if the cpu is idle and when it
> entered the idle state, correct ?
>


Exactly.

 
> If the cpu is idle and the information is per cpu, how will you read
> this value from another cpu without introducing a locking mechanism ?
> 


I think it might be tolerated for incoherency of that data. Governor reads the
data only, and if recoded start time or end time are different in few usec with
real one then it doesn't effect to the result much.


> Does it mean the cpufreq governor needs cpuidle ? I am wondering if
> these informations shouldn't be retrieved from the scheduler, not from
> cpuidle.
> 


Yes, tick_sched per-cpu variable has all information that I need. But it isn't
global variable. And I'm afraid to change static variable to global one as my
pleases.


Thanks,
Jonghwa

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