linux-kernel - Re: [PATCH 4/4] cpuidle/menu: add per cpu pm_qos_resume

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Date:   Fri, 23 Sep 2016 12:58:40 +0800
From:   Alex Shi <alex.shi@...aro.org>
To:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        open list <linux-kernel@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 4/4] cpuidle/menu: add per cpu pm_qos_resume_latency
 consideration



On 09/23/2016 09:36 AM, Rafael J. Wysocki wrote:
> On 9/14/2016 10:28 AM, Vincent Guittot wrote:
>> Hi Rafael,
>>
>> On 14 September 2016 at 00:10, Rafael J. Wysocki <rjw@...ysocki.net>
>> wrote:
>>> On Tuesday, September 13, 2016 10:02:49 PM Alex Shi wrote:
>>>> Hi Daniel & Rafael,
>>>>
>>>> Any comments on this patch?
>>> I actually am not sure about the whole series.
>>>
>>> I know your motivation, but honestly the changes here may not be the
>>> best way
>>> to achieve what you need.

Is there some idea for better way?

>>>
>>> You may think that the changes are trivial, but in fact they are
>>> not.  There
>>> are side effects and I'm not sure about the resulting user space
>>> interface
>>> at all.

What's concern is abuse of user interface? If the request come from user
level, is there other ways to set a value for this?


>> This patchset has got 2 parts:
>> - one part is about taking into account per-device resume latency
>> constraint when selecting the idle state of a CPU. This value can
>> already be set by kernel (even if it's probably not done yet) but this
>> constraint is never taken into account
>> - the other part is about exposing the resume latency to userspace.
>> This part might raise more discussion but I see one example that could
>> take advantage of this. When you have several clusters of CPUs and you
>> want to dedicate some CPUs  to latency sensitive activity and prevent
>> deep sleep  state on these CPUs but you want to let the other CPUs
>> using all C-state
> 
> The first very basic question about this I have is whether or not the
> device PM QoS mechanism is suitable for the task at hand at all.
> 
> It certainly hasn't been invented with it in mind.
> 

Look though the device PM QoS, this kind of usage seems sensible. So why
not?

Regards
Alex