Subject: Re: [PATCH V2] cpuidle/governors: Fix logic in selection of idle states

From: Daniel Lezcano <[email-masked]>
Date: Tue, 28 Jan 2014 09:46:39 +0100

On 01/24/2014 11:21 AM, Preeti U Murthy wrote:
> On 01/24/2014 02:38 PM, Daniel Lezcano wrote:
>> On 01/23/2014 12:15 PM, Preeti U Murthy wrote:
>>> Hi Daniel,
>>>
>>> Thank you for the review.

[ ... ]

>>> ---
>>>    drivers/cpuidle/cpuidle.c          |   15 +++++
>>>    drivers/cpuidle/governors/ladder.c |  101
>>> ++++++++++++++++++++++++++----------
>>>    drivers/cpuidle/governors/menu.c   |    7 +-
>>>    3 files changed, 90 insertions(+), 33 deletions(-)
>>>
>>> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
>>> index a55e68f..19d17e8 100644
>>> --- a/drivers/cpuidle/cpuidle.c
>>> +++ b/drivers/cpuidle/cpuidle.c
>>> @@ -131,8 +131,9 @@ int cpuidle_idle_call(void)
>>>
>>>        /* ask the governor for the next state */
>>>        next_state = cpuidle_curr_governor->select(drv, dev);
>>> +
>>> +    dev->last_residency = 0;
>>>        if (need_resched()) {
>>
>> What about if (need_resched() || next_state < 0) ?
>
> Hmm.. I feel we need to distinguish between the need_resched() scenario
> and the scenario when no idle state was suitable through the trace
> points at-least.

Well, I don't think so as soon as we don't care about the return value 
of cpuidle_idle_call in both cases.

The traces are following a specific format. That is if the state is -1 
(PWR_EVENT_EXIT), it means exiting the current idle state.

The idlestat tool [1] is using this traces to open - close transitions.

IMO, if the cpu is not entering idle, it should just exit without any 
idle traces.

This portion of code is a bit confusing because it is introduced by the 
menu governor updates post-poned when entering the next idle state (not 
exiting the current idle state with good reasons).

   -- Daniel

[1] http://git.linaro.org/power/idlestat.git

> This could help while debugging when we could find situations where
> there are no tasks to run, yet the cpu is not entering any idle state.
> The traces could help clearly point that no idle state was thought
> suitable by the governor. Of course there are many other means to find
> this out, but this seems rather straightforward. Hence having the
> condition next_state < 0 between trace_cpu_idle*() would be apt IMHO.
>
> Regards
> Preeti U Murthy
>
>>
>>> -        dev->last_residency = 0;
>>>            /* give the governor an opportunity to reflect on the
>>> outcome */
>>>            if (cpuidle_curr_governor->reflect)
>>>                cpuidle_curr_governor->reflect(dev, next_state);
>>> @@ -141,6 +142,16 @@ int cpuidle_idle_call(void)
>>>        }
>>>
>>>        trace_cpu_idle_rcuidle(next_state, dev->cpu);
>>> +    /*
>>> +     * NOTE: The return code should still be success, since the
>>> verdict of
>>> +     * this call is "do not enter any idle state". It is not a failed
>>> call
>>> +     * due to errors.
>>> +     */
>>> +    if (next_state < 0) {
>>> +        entered_state = next_state;
>>> +        local_irq_enable();
>>> +        goto out;
>>> +    }
>>>
>>>        broadcast = !!(drv->states[next_state].flags &
>>> CPUIDLE_FLAG_TIMER_STOP);
>>>
>>> @@ -156,7 +167,7 @@ int cpuidle_idle_call(void)
>>>        if (broadcast)
>>>            clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_EXIT, &dev->cpu);
>>>
>>> -    trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, dev->cpu);
>>> +out:    trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, dev->cpu);
>>>
>>>        /* give the governor an opportunity to reflect on the outcome */
>>>        if (cpuidle_curr_governor->reflect)
>

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