linux-kernel - Re: [PATCH 09/16] sched: normalize tg load contributions against runnable time

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Date:	Fri, 06 Jul 2012 14:23:59 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Paul Turner <pjt@...gle.com>
Cc:	linux-kernel@...r.kernel.org, Venki Pallipadi <venki@...gle.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ben Segall <bsegall@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Subject: Re: [PATCH 09/16] sched: normalize tg load contributions against
 runnable time

On Wed, 2012-06-27 at 19:24 -0700, Paul Turner wrote:

> +       contrib = div_u64(sa->runnable_avg_sum << 12,
> +                         sa->runnable_avg_period + 1);

> +
> +       /*
> +        * Unlike a task-entity, a group entity may be using >=1 cpu globally.
> +        * However, in the case that it's using <1 cpu we need to form a
> +        * correction term so that we contribute the same load as a task of
> +        * equal weight. (Global runnable time is taken as a fraction over
> +        * 2^12.)
> +        */
> +       runnable_avg = atomic_read(&tg->runnable_avg);
> +       if (runnable_avg < (1<<12)) {
> +               se->avg.load_avg_contrib *= runnable_avg;
> +               se->avg.load_avg_contrib /= (1<<12);
> +       }


Did we really have to open-code a 4096 fixed point here? Couldn't we
re-use some of the existing _SCALE things.. if not why 12?


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