linux-kernel - Re: [RFCv5 PATCH 25/46] sched: Add over-utilization/tipping point indicator

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Date:	Tue, 29 Sep 2015 13:08:46 -0700
From:	Steve Muckle <steve.muckle@...aro.org>
To:	Morten Rasmussen <morten.rasmussen@....com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...hat.com, vincent.guittot@...aro.org,
	daniel.lezcano@...aro.org,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	yuyang.du@...el.com, mturquette@...libre.com, rjw@...ysocki.net,
	Juri Lelli <Juri.Lelli@....com>, sgurrappadi@...dia.com,
	pang.xunlei@....com.cn, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [RFCv5 PATCH 25/46] sched: Add over-utilization/tipping point
 indicator

On 08/14/2015 06:02 AM, Morten Rasmussen wrote:
> To be sure not to break smp_nice, we have defined over-utilization as
> when:
> 
> cpu_rq(any)::cfs::avg::util_avg + margin > cpu_rq(any)::capacity
> 
> is true for any cpu in the system. IOW, as soon as one cpu is (nearly)
> 100% utilized, we switch to load_avg to factor in priority.
> 
> Now with this definition, we can skip periodic load-balance as no cpu
> has an always-running task when the system is not over-utilized. All
> tasks will be periodic and we can balance them at wake-up. This
> conservative condition does however mean that some scenarios that could
> benefit from energy-aware decisions even if one cpu is fully utilized
> would not get those benefits.
> 
> For system where some cpus might have reduced capacity on some cpus
> (RT-pressure and/or big.LITTLE), we want periodic load-balance checks as
> soon a just a single cpu is fully utilized as it might one of those with
> reduced capacity and in that case we want to migrate it.
> 
> I haven't found any reasonably easy-to-track conditions that would work
> better. Suggestions are very welcome.

Workloads with a single heavy task and many small tasks are pretty
common. I'm worried about the single heavy task tripping the
over-utilization condition on a b.L system, EAS getting turned off, and
small tasks running on big CPUs, leading to an increase in power
consumption.

Perhaps an extension to the over-utilization logic such as the following
could cause big CPUs being saturated by a single task to be ignored?

util(cpu X) + margin > capacity(cpu X) &&
  (capacity(cpu X) != max_capacity ? 1 : nr_running(cpu X) > 1)
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