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

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Message-ID: <68689fcb-3cb8-4685-58ef-0bec84be2047@arm.com>
Date:   Mon, 6 Aug 2018 12:08:07 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Quentin Perret <quentin.perret@....com>
Cc:     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 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.
Task load (task_h_load(p)) is used in 
wake_affine()->wake_affine_weight() but for this to be called it has to 
be a wakeup (SD_BALANCE_WAKE).

> The utilization of this new task is then set to half of the remaining
> capacity of the selected CPU which means that the idlest you are, the
> biggest the task will be initialized to. This can easily be half a big
> core which can be bigger than the max capacity of a little like on
> hikey960. Then, util_est  will keep track of this value for a while
> which will make this task like a big one.

[...]