linux-kernel - Re: [PATCH V3 2/6] sched: idle: cpuidle: Check the latency req before idle

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Message-ID: <545DF30B.1050305@linux.vnet.ibm.com>
Date:	Sat, 08 Nov 2014 16:10:11 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>, 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, lenb@...nel.org
Subject: Re: [PATCH V3 2/6] sched: idle: cpuidle: Check the latency req before
 idle

On 11/07/2014 08:01 PM, 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.
> 
> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> Acked-by: Nicolas Pitre <nico@...aro.org>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Reviewed-by: Len Brown <len.brown@...el.com>
> ---

Reviewed-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>

Regards
Preeti U Murthy

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