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

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Date:   Fri, 3 Aug 2018 09:18:52 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
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>,
        Dietmar Eggemann <dietmar.eggemann@....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 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'm not really concerned about re-enabling load balance but more that
> the effort of packing of tasks in few cpus/clusters that EAS tries to
> do can be broken for every new task.

Well, re-enabling load balance immediately would break the nice placement
that EAS found, because it would shuffle all tasks around and break the
packing strategy. Letting that sole new task go in find_idlest_cpu()
shouldn't impact the placement of existing tasks. That might have an energy
cost for that one task, yes, but it's really hard to do anything smarter
with new tasks IMO ... EAS simply can't work without a utilization value.

> So I wonder what is better for EAS : Make sure to efficiently spread
> newly created tasks in cas of fork bomb or  try to not break EAS task
> placement with every newly created tasks

That shouldn't break the placement per se, we're just making one
temporary exception for new tasks. What do you think 'the right thing'
to do is ? To just put new tasks on prev_cpu or something like that ?

That might help some use-cases I suppose, but will probably harm others ...
I'm just not too keen on making assumptions about the size of new tasks,
that's all. But I'm definitely open to ideas if there is something
smarter we can do.

Thanks,
Quentin