linux-kernel - Re: [PATCH v5 09/14] sched: Add over-utilization/tipping point indicator

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Message-ID: <454ee0da-9c6c-6944-25e5-0fbbf89bc3b6@arm.com>
Date:   Mon, 6 Aug 2018 15:20:27 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Quentin Perret <quentin.perret@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Chris Redpath <chris.redpath@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        viresh kumar <viresh.kumar@...aro.org>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        "Cc: Steve Muckle" <smuckle@...gle.com>, adharmap@...cinc.com,
        "Kannan, Saravana" <skannan@...cinc.com>, pkondeti@...eaurora.org,
        Juri Lelli <juri.lelli@...hat.com>,
        Eduardo Valentin <edubezval@...il.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        currojerez@...eup.net, Javi Merino <javi.merino@...nel.org>
Subject: Re: [PATCH v5 09/14] sched: Add over-utilization/tipping point
 indicator

On 08/06/2018 02:37 PM, Vincent Guittot wrote:
> On Mon, 6 Aug 2018 at 14:29, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>
>> On 08/06/2018 12:33 PM, Vincent Guittot wrote:
>>> On Mon, 6 Aug 2018 at 12:08, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>
>>>> On 08/06/2018 10:40 AM, Vincent Guittot wrote:
>>>>> On Fri, 3 Aug 2018 at 17:55, Quentin Perret <quentin.perret@....com> wrote:
>>>>>>
>>>>>> On Friday 03 Aug 2018 at 15:49:24 (+0200), Vincent Guittot wrote:
>>>>>>> On Fri, 3 Aug 2018 at 10:18, Quentin Perret <quentin.perret@....com> wrote:
>>>>>>>>
>>>>>>>> On Friday 03 Aug 2018 at 09:48:47 (+0200), Vincent Guittot wrote:
>>>>>>>>> On Thu, 2 Aug 2018 at 18:59, Quentin Perret <quentin.perret@....com> wrote:
>>>>
>>>> [...]
>>>>
>>>>>> I think we're discussing two different things right now:
>>>>>>        1. Should forkees go in find_energy_efficient_cpu() ?
>>>>>>        2. Should forkees have 0 of initial util_avg when EAS is enabled ?
>>>>>
>>>>> It's the same topic: How EAS should consider a newly created task ?
>>>>>
>>>>> For now, we let the "performance" mode selects a CPU. This CPU will
>>>>> most probably be worst CPU from a EAS pov because it's the idlest CPU
>>>>> in the idlest group which is the opposite of what EAS tries to do
>>>>>
>>>>> The current behavior is :
>>>>> For every new task, the cpu selection is done assuming it's a heavy
>>>>> task with the max possible load_avg,  and it looks for the idlest cpu.
>>>>> This means that if the system is lightly loaded, scheduler will select
>>>>> most probably a idle big core.
>>>>
>>>> AFAICS, task load doesn't seem to be used for find_idlest_cpu() (
>>>> find_idlest_group() and find_idlest_group_cpu()). So the forkee
>>>> (SD_BALANCE_FORK) is placed independently of his task load.
>>>
>>> hmm ... so what is used if load or runnable load are not used ?
>>> find_idlest_group() uses load and runnable load but skip spare
>>> capacity in case of fork
>>
>> Yes, runnable load and load are used, but from the cpus, not from the task.
> 
> yes that's right, I have skipped the "task" word when reading.
> So scheduler looks for the idlest CPU taking into account only CPU
> loads. Then the task load starts to highest value until it get a
> chance to reduce and stabilize to its final value

This could potentially allow us to find a better init value for 
sa->[runnable]_load_avg. At least we could use the information of the 
initial task rq.

> 
>>
>> [...]