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

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Date:	Fri, 07 Nov 2014 10:35:00 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
CC:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	patches@...aro.org
Subject: Re: [PATCH V2 1/5] sched: idle: cpuidle: Check the latency req before
 idle

On 11/07/2014 05:29 AM, Preeti U Murthy wrote:
> On 11/06/2014 07:12 PM, Daniel Lezcano wrote:
>>
>> Preeti,
>>
>> I am wondering if we aren't going to a false debate.
>>
>> If the latency_req is 0, we should just poll and not enter in any idle
>> state even if one has zero exit latency. With a zero latency req, we
>> want full reactivity on the system, not enter an idle state with all the
>> computation in the menu governor, no ?
>>
>> I agree this patch changes the behavior on PowerPC, but only if the
>> latency_req is set to zero. I don't think we are worried about power
>> saving when setting this value.
>>
>> Couldn't the patch accepted as it is for the sake of consistency on all
>> the platform and then we optimize cleanly for the special latency zero
>> case ?
>
> Alright Daniel, you can go ahead. I was thinking this patch through and
> now realize that, like you point out the logic will only get complicated
> with all the additional hack.
>
> But would it be possible to add the weak arch_cpu_idle_loop() call for
> the cases where latency requirement is 0 like you had suggested earlier
> ? This would ensure the polling logic does not break on PowerPC and we
> don't bother the governor even. I will add the function in the core
> PowerPC code. If arch does not define this function it will fall back to
> cpu_idle_loop(). Fair enough?

Yes, sounds good.

I will add the weak function as the first patch in the series.

Thanks for your reviews.

   -- Daniel

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