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

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Date:	Wed, 04 Jul 2012 21:48:05 +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:
> Entities of equal weight should receive equitable distribution of cpu time.
> This is challenging in the case of a task_group's shares as execution may be
> occurring on multiple cpus simultaneously.
> 
> To handle this we divide up the shares into weights proportionate with the load
> on each cfs_rq.  This does not however, account for the fact that the sum of
> the parts may be less than one cpu and so we need to normalize:
>   load(tg) = min(runnable_avg(tg), 1) * tg->shares
> Where runnable_avg is the aggregate time in which the task_group had runnable
> children. 

I remember we had a bit of a discussion on this last time, I thought you
were going to convince me this approximation was 'right'.

Care to still do so.. the rationale used should at least live in a
comment somewhere, otherwise someone will go silly trying to understand
things later on.

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