linux-kernel - Re: [PATCH V2 1/5] sched: idle: cpuidle: Check the latency req before idle

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Date:	Wed, 05 Nov 2014 22:41:00 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	nicolas.pitre@...aro.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Subject: Re: [PATCH V2 1/5] sched: idle: cpuidle: Check the latency req before
 idle

On 11/05/2014 10:57 PM, Rafael J. Wysocki wrote:
> On Thursday, October 23, 2014 11:01:17 AM Daniel Lezcano wrote:
>> When the pmqos latency requirement is set to zero that means "poll in all the
>> cases".
>>
>> That is correctly implemented on x86 but not on the other archs.
>>
>> As how is written the code, if the latency request is zero, the governor will
>> return zero, so corresponding, for x86, to the poll function, but for the
>> others arch the default idle function. For example, on ARM this is wait-for-
>> interrupt with a latency of '1', so violating the constraint.
>>
>> In order to fix that, do the latency requirement check *before* calling the
>> cpuidle framework in order to jump to the poll function without entering
>> cpuidle. That has several benefits:
>>
>>   1. It clarifies and unifies the code
>>   2. It fixes x86 vs other archs behavior
>>   3. Factors out the call to the same function
>>   4. Prevent to enter the cpuidle framework with its expensive cost in
>>      calculation
>>
>> As the latency_req is needed in all the cases, change the select API to take
>> the latency_req as parameter in case it is not equal to zero.
>>
>> As a positive side effect, it introduces the latency constraint specified
>> externally, so one more step to the cpuidle/scheduler integration.
>
> I'm expecting to see a new version of this patchset relatively soon.
>
> Are you planning to send one?

I would like to find an agreement with Preeti. But, yes, I am on it.

   -- Daniel

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