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Date:   Wed, 7 Mar 2018 13:24:37 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Patrick Bellasi <patrick.bellasi@arm.com>
Cc:     linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Paul Turner <pjt@google.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Todd Kjos <tkjos@android.com>,
        Joel Fernandes <joelaf@google.com>,
        Steve Muckle <smuckle@google.com>
Subject: Re: [PATCH v5 1/4] sched/fair: add util_est on top of PELT
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On Wed, Mar 07, 2018 at 11:31:49AM +0000, Patrick Bellasi wrote:
> > It appears to me this isn't a stable situation and completely relies on
> > the !nr_running case to recalibrate. If we ensure that doesn't happen
> > for a significant while the sum can run-away, right?
> 
> By away you mean go over 1024 or overflow the unsigned int storage?

the later, I think you can make it arbitrarily large. Have a busy task
on CPU0, this ensure !nr_running never happens.

Start a busy task on CPU1, wait for it to hit u=1, then migrate it to
CPU0, then wait for it to hit u=.5 then kill it, this effectively adds
.5 to the enqueued value, repeat indefinitely.