Re: [RFC PATCH 3/3] idle: store the idle state index in the struct rq

[Daniel Lezcano <daniel.lezcano@linaro.org>]

On 01/31/2014 04:50 PM, Arjan van de Ven wrote:
> On 1/31/2014 7:37 AM, Daniel Lezcano wrote:
>> On 01/31/2014 04:07 PM, Arjan van de Ven wrote:
>>>>>>
>>>>>> Hence I think this patch would make sense only with additional
>>>>>> information
>>>>>> like exit_latency or target_residency is present for the scheduler.
>>>>>> The idle
>>>>>> state index alone will not be sufficient.
>>>>>
>>>>> Alternatively, can we enforce sanity on the cpuidle infrastructure to
>>>>> make the index naturally ordered? If not, please explain why :-)
>>>>
>>>> The commit id 71abbbf856a0e70 says that there are SOCs which could have
>>>> their target_residency and exit_latency values change at runtime. This
>>>> commit thus removed the ordering of the idle states according to their
>>>> target_residency/exit_latency. Adding Len and Arjan to the CC.
>>>
>>> the ARM folks wanted a dynamic exit latency, so.... it makes much more
>>> sense
>>> to me to store the thing you want to use (exit latency) than the number
>>> of the state.
>>>
>>> more than that, you can order either by target residency OR by exit
>>> latency,
>>> if you sort by one, there is no guarantee that you're also sorted by the
>>> other
>>
>> IMO, it would be preferable to store the index for the moment as we
>> are integrating cpuidle with the scheduler. The index allows to access
>> more informations. Then when
>> everything is fully integrated we can improve the result, no ?
>
> more information, yes. but if the information isn't actually accurate
> (because it keeps changing
> in the datastructure away from what it was for the cpu)... are you
> really achieving what you want?
>
> on x86 I don't care; we don't actually change these dynamically much[1].
> But if you have 1 or 2 things in mind to use,
> I would suggest copying those 2 integers instead as we go, rather than
> the index.
> Saves refcounting/locking etc etc nightmare as well on the other
> subsystems' datastructures..
> ... which you likely need to do to actually follow that index.

Hmm, yeah. That's a fair argument. That is true, the races and 
locks/refcnt are something we have to worried about. But also we may 
want to prevent duplicating the data across the subsystems.

> [1] Although in an ACPI world, the total number of C states can vary,
> for example it used to be quite common
> that you got an extra C state on battery versus on wall power. This sort
> of dynamic thing requires refcounting
> if more than the local cpuidle uses the data structures.
>


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