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 18:37:07 +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 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) :) 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?

Thanks,
Jonghwa.

> Thanks
>   -- Daniel
> 


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