linux-kernel - Re: [PATCH 1/3] cpuidle: Store the idle start time stamp

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Date:	Wed, 15 Apr 2015 14:50:33 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	rjw@...ysocki.net, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, nicolas.pitre@...aro.org
Subject: Re: [PATCH 1/3] cpuidle: Store the idle start time stamp

On 04/15/2015 02:42 PM, Peter Zijlstra wrote:
> On Wed, Apr 15, 2015 at 02:29:06PM +0200, Daniel Lezcano wrote:
>> On 04/15/2015 12:20 PM, Peter Zijlstra wrote:
>>> On Wed, Apr 15, 2015 at 12:00:22PM +0200, Daniel Lezcano wrote:
>>>> +	target_state->idle_stamp = ktime_to_us(ktime_get());
>>>
>>> ktime_get_ns();
>>>
>>
>> Hmm, sounds like I missed we are dealing with different units (us / ns) in
>> cpuidle / sched.
>>
>> Would it make sense to store the time into a ktime structure instead and use
>> the relevant function depending on the place we are inspecting the value
>> from ?
>
> Why would you ever want to use us? Does ARM enjoy calculating /1000?
>
> Ah, I see the !scalar ktime got whacked, good! At which point the u64 ns
> and ktime are the same. That said I prefer the u64 over ktime because
> its easier to manipulate.

Ok. I was trying to save one variable on the stack by reusing the 
idle_stamp value but if we store it in a different unit, then we have to 
keep it in order to have usecond for the governors.

I was thinking about converting to nanosecond the cpuidle framework but 
it is not worth to do that as the resolution is too high for the idle 
states.




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