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

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Message-ID: <CAKohpong15qzFg_sZVB77wVm4gu-4Se1E=eqvJOv96fgnyMXDA@mail.gmail.com>
Date:	Thu, 26 Sep 2013 11:54:09 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Daniel Lezcano <daniel.lezcano@...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 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?


> 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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