linux-kernel - Re: [PATCH v9 04/10] sched: Make sched entity usage tracking scale-invariant

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Message-ID: <20141121123544.GE23177@e105550-lin.cambridge.arm.com>
Date:	Fri, 21 Nov 2014 12:35:44 +0000
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"kamalesh@...ux.vnet.ibm.com" <kamalesh@...ux.vnet.ibm.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"efault@....de" <efault@....de>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH v9 04/10] sched: Make sched entity usage tracking
 scale-invariant

On Mon, Nov 03, 2014 at 04:54:41PM +0000, Vincent Guittot wrote:
> From: Morten Rasmussen <morten.rasmussen@....com>
> 
> Apply frequency scale-invariance correction factor to usage tracking.

s/usage/utilization/

> Each segment of the running_load_avg geometric series is now scaled by the
> current frequency so the utilization_avg_contrib of each entity will be

s/entity/sched_entity/

> invariant with frequency scaling. As a result, utilization_load_avg which is
> the sum of utilization_avg_contrib, becomes invariant too. So the usage level

s/sum of utilization_avg_contrib/sum of sched_entity
utilization_avg_contribs/

s/usage/utilization/

> that is returned by get_cpu_usage, stays relative to the max frequency as the
> cpu_capacity which is is compared against.

The last bit doesn't parse right. '... Maybe it is better to drop
the reference to get_cpu_usage which hasn't been defined yet and rewrite
the thing to:

Apply frequency scale-invariance correction factor to utilization
tracking. Each segment of the running_load_avg geometric series is now
scaled by the current frequency so the utilization_avg_contrib of each
entity will be invariant with frequency scaling. As a result,
utilization_load_avg which is the sum of sched_entity
utilization_avg_contribs becomes invariant too and is now relative to
the max utilization at the max frequency (=cpu_capacity).

I think we should add:

arch_scale_freq_capacity() is reintroduced to provide the frequency
compensation scaling factor.

> Then, we want the keep the load tracking values in a 32bits type, which implies

s/Then, we/We/

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