linux-kernel - Re: [RFC/RFT][PATCH 4/7] cpuidle: menu: Split idle duration prediction from state selection

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Message-ID: <73d0ffca-7496-12f3-e1b7-a96494507817@linux.intel.com>
Date:   Tue, 6 Mar 2018 22:07:01 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Ilsche <thomas.ilsche@...dresden.de>,
        Doug Smythies <dsmythies@...us.net>,
        Rik van Riel <riel@...riel.com>,
        Mike Galbraith <mgalbraith@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RFC/RFT][PATCH 4/7] cpuidle: menu: Split idle duration
 prediction from state selection

On 2018/3/6 16:45, Peter Zijlstra wrote:
> On Tue, Mar 06, 2018 at 10:15:10AM +0800, Li, Aubrey wrote:
>> On 2018/3/5 21:53, Peter Zijlstra wrote:
>>> On Mon, Mar 05, 2018 at 02:05:10PM +0100, Rafael J. Wysocki wrote:
>>>> On Mon, Mar 5, 2018 at 1:50 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>>>>> On Mon, Mar 05, 2018 at 12:47:23PM +0100, Rafael J. Wysocki wrote:
>>>
>>>>>> IOW, the target residency of the selected state doesn't tell you how
>>>>>> much time you should expect to be idle in general.
>>>>>
>>>>> Right, but I think that measure isn't of primary relevance. What we want
>>>>> to know is: 'should I stop the tick' and 'what C state do I go to'.
>>
>> I understood the benefit of mapping duration to state number, is duration <->
>> state number mapping a generic solution to all arches?
> 
> Yes, all platforms have a limited set of possible idle states.
> 
>> Back to the user's concern is, "I'm running a latency sensitive application, and
>> I want idle switching ASAP". So I think the user may not care about what C state
>> to go into, that is, even if a deeper state has chance to go, the user striving
>> for a higher workload score may still not want it?
> 
> The user caring about performance very much cares about the actual idle
> state too, exit latency for deeper states is horrific and will screw
> them up just as much as the whole nohz timer reprogramming does.
> 
> We can basically view the whole nohz thing as an additional entry/exit
> latency for the idle state, which is why I don't think its weird to
> couple them.

okay, I see your point now. Thanks!

> 
>>>> Maybe just return a "nohz" indicator from cpuidle_select() in addition
>>>> to the state index and make the decision in the governor?
>>>
>>> Much better option than returning a duration :-)
>>>
>> So what does "nohz = disable and state index = deepest" mean? This combination
>> does not make sense for performance only purpose?
> 
> I tend to agree with you that the state space allowed by a separate
> variable is larger than required, but it's significantly smaller than
> preserving 'time' so I can live with it.
>