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

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Message-ID: <43825f88-762e-11e5-d70a-45152b95599d@linux.intel.com>
Date:   Tue, 6 Mar 2018 10:15:10 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "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 2018/3/5 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? 

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?

>>>
>>> In order to answer those questions we need durations as input, but I
>>> don't think we should preserve durations throughout. The scheme from the
>>> above link reduces to N states in order to deal with arbitrary
>>> distributions, only the actual states -- ie boundaries where our answers
>>> changes -- are relevant, anything inside those boundaries would lead to
>>> the exact same answer anyway.
>>
>> I generally agree here, but I'm not convinced about flagging the
>> states, splitting them and so on.
> 
> I think linking them like that makes sense, but I can see room for
> discussion...
> 
>> 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?

Thanks,
-Aubrey