linux-kernel - Re: [PATCH 3/3] cpuidle/menu: add per cpu pm_qos_resume

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Date:   Mon, 23 Jan 2017 22:58:28 +0800
From:   Alex Shi <alex.shi@...aro.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 3/3] cpuidle/menu: add per cpu pm_qos_resume_latency
 consideration



On 01/23/2017 08:50 PM, Daniel Lezcano wrote:
> On Sun, Jan 22, 2017 at 09:31:44AM +0800, Alex Shi wrote:
>>
>>> Yeah, that could be problematic. The code snippet gives the general idea but it
>>> could be changed by for example by a flag telling the cpus when they enter idle
>>> to update their state_count. Or something like that.
>>
>> Yes, this idea could be helpful.
>>
>> But since the idle path isn't a hot path. and a few memory access won't cost
>> a lot. So I doubt if the benefit could be measurable.
> 
> It won't be measurable, as well as reading the cpu device latency before
> checking the latency req is zero, but it makes sense.

Just simple change the cpu state may make it looks unnatural. :)
> 
> The idle routine is not a hot path but a very special place where the interrupt
> are disabled, the rcu is not usable, tick is disabled etc ...
> 
> Perhaps it is not a problem for the moment, but it is probably worth to mention that
> using API from other subsystems in the idle select path could be problematic
> and perhaps it is time to think about another approach for the future.
> 

Yes, before idle, it did consider lots of parts, included pm qos for
long time... :)