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 22:57:47 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
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
Subject: Re: [PATCH V2 1/5] sched: idle: cpuidle: Check the latency req before idle

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 speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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