linux-kernel - Re: [PATCH v3 09/12] Revert "sched: Put rq's sched_avg under CONFIG_FAIR_GROUP

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

On 11 July 2014 18:13, Morten Rasmussen <morten.rasmussen@....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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