linux-kernel - Re: [PATCH 20/21] cpuidle: don't calculate time-diff if entered

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Date:	Thu, 26 Sep 2013 10:25:35 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Patch Tracking <patches@...aro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 20/21] cpuidle: don't calculate time-diff if entered_state
 == 0

On 09/26/2013 08:24 AM, Viresh Kumar wrote:
> On 26 September 2013 04:08, Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
>> On 09/22/2013 03:21 AM, Viresh Kumar wrote:
>>> If entered_state == 0, we don't need to set dev->last_residency to 'diff' as we
>>> will be setting it to zero without using its new value.
>>
>> I don't get it, can you elaborate.
>
> Sure.. We have following code in cpuidle_enter_state():
>
> ---------
>          diff = ktime_to_us(ktime_sub(time_end, time_start));
>          if (diff > INT_MAX)
>                  diff = INT_MAX;
>
>          dev->last_residency = (int) diff;
>
>          if (entered_state >= 0) {
>                  /* Update cpuidle counters */
>                  /* This can be moved to within driver enter routine
>                   * but that results in multiple copies of same code.
>                   */
>                  dev->states_usage[entered_state].time += dev->last_residency;
>                  dev->states_usage[entered_state].usage++;
>          } else {
>                  dev->last_residency = 0;
>          }
> -------
>
> We are setting last_residency to 0 when (entered_state < 0) and aren't using
> the value of "diff". So, we can actually skip calculations of time_end, diff and
> last_residency when (entered_state < 0).. Makes sense?

Yes I agree, but why are you saying "If entered_state == 0, we don't 
need to ..." ?

>> We can be a long time in this state
>> (eg. if the prediction is false).
>
> Okay.. I didn't get it :)
>


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