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From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Tue, 15 Jul 2014 11:27:19 +0200
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Subject: Re: [PATCH v3 09/12] Revert "sched: Put rq's sched_avg under CONFIG_FAIR_GROUP_SCHED"
To: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Russell King - ARM Linux <linux@arm.linux.org.uk>,
        LAK <linux-arm-kernel@lists.infradead.org>,
        Preeti U Murthy <preeti@linux.vnet.ibm.com>,
        Mike Galbraith <efault@gmx.de>,
        Nicolas Pitre <nicolas.pitre@linaro.org>,
        "linaro-kernel@lists.linaro.org" <linaro-kernel@lists.linaro.org>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Dietmar Eggemann <Dietmar.Eggemann@arm.com>
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On 11 July 2014 18:13, Morten Rasmussen <morten.rasmussen@arm.com> wrote:

[snip]

>
> In this example using rq->avg leads to imbalance whereas unweighted load
> would not. Correct me if I missed anything.

You just miss to take into account how the imbalance is computed

>
> Coming back to the previous example. I'm not convinced that inflation of
> the unweighted load sum when tasks overlap in time is a bad thing. I
> have mentioned this before. The average cpu utilization over the 40ms
> period is 50%. However the true compute capacity demand is 200% for the
> first 15ms of the period, 100% for the next 5ms and 0% for the remaining
> 25ms. The cpu is actually overloaded for 15ms every 40ms. This fact is
> factored into the unweighted load whereas rq->avg would give you the
> same utilization no matter if the tasks are overlapped or not. Hence
> unweighted load would give us an indication that the mix of tasks isn't
> optimal even if the cpu has spare cycles.
>
> If you don't care about overlap and latency, the unweighted sum of task
> running time (that Peter has proposed a number of times) is better
> metric, IMHO. As long the cpu isn't fully utilized.
>
> Morten
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