linux-kernel - Re: [PATCH v2 08/11] sched: get CPU's activity statistic

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Date:	Wed, 4 Jun 2014 12:55:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"efault@....de" <efault@....de>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v2 08/11] sched: get CPU's activity statistic

On Wed, Jun 04, 2014 at 11:36:19AM +0100, Morten Rasmussen wrote:
> On Wed, Jun 04, 2014 at 11:17:24AM +0100, Peter Zijlstra wrote:
> > Let me explain the 75%, take any one of the above scenarios. Lets call
> > the two tasks A and B, and let for a moment assume A always wins and
> > runs first, and then B.
> > 
> > So A will be runnable for 25%, B otoh will be runnable the entire time A
> > is actually running plus its own running time, giving 50%. Together that
> > makes 75%.
> > 
> > If you release the assumption that A runs first, but instead assume they
> > equally win the first execution, you get them averaging at 37.5% each,
> > which combined will still give 75%.
> 
> But that is assuming that the first task gets to run to completion of it
> busy period. If it uses up its sched_slice and we switch to the other
> tasks, they both get to wait.
> 
> For example, if the sched_slice is 5 ms and the busy period is 10 ms,
> the execution pattern would be: A, B, A, B, idle, ... In that case A is
> runnable for 15 ms and B is for 20 ms. Assuming that the overall period
> is 40 ms, the A runnable is 37.5% and B is 50%.

Indeed, with preemption added you can pull this out further. You can
then indeed get infinitesimally close to 100% with this.

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