Re: [RFC PATCH 1/2] cpuidle: Add idle enter/exit time stamp for notifyingcurrent idle state.

[Daniel Lezcano]

On 04/02/2013 11:37 AM, jonghwa3.lee@xxxxxxxxxxx wrote:
> On 2013ë 04ì 02ì 16:34, Daniel Lezcano wrote:
> 
>> On 04/02/2013 08:17 AM, jonghwa3.lee@xxxxxxxxxxx wrote:
>>> On 2013ë 04ì 02ì 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@xxxxxxxxxxx>
>>>>> ---
>>>>
>>>> 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.

> 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 ?

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 ?

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

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