linux-kernel - Re: [PATCH 2/2 v4] sched: Rewrite per entity runnable load average tracking

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Date:	Tue, 29 Jul 2014 17:55:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>,
	Benjamin Segall <bsegall@...gle.com>,
	arjan.van.de.ven@...el.com, Len Brown <len.brown@...el.com>,
	rafael.j.wysocki@...el.com, alan.cox@...el.com,
	"Gross, Mark" <mark.gross@...el.com>,
	"fengguang.wu@...el.com" <fengguang.wu@...el.com>
Subject: Re: [PATCH 2/2 v4] sched: Rewrite per entity runnable load average
 tracking

On Tue, Jul 29, 2014 at 03:35:10PM +0200, Peter Zijlstra wrote:
> On Tue, Jul 29, 2014 at 09:53:44AM +0800, Yuyang Du wrote:
> > On Tue, Jul 29, 2014 at 11:39:11AM +0200, Peter Zijlstra wrote:
> > > > > For task, assuming its load.weight does not change much, yes, we can. But in theory, task's
> > > > 
> > > > I would even say that the load_avg of a task should not be impacted by
> > > > an old priority value. Once, the priority of a task is changed, we
> > > > should only take into account this new priority to weight the load_avg
> > > > of the task
> > > 
> > > So for tasks I would immediately agree, and I think for groups too,
> > > seeing how the group weight is based off of this avg, if you then
> > > include the old weight we'll get a feedback loop. This might not be
> > > desired as it would counteract the SMP movement of tasks.
> > 
> > Including the old weight can we get the *right* feedback. Because say until
> > weight is changed, we are balanced, changed weight leads to imbalance. Without
> > old weight, the imbalance is multiplied by the history, like we have never been
> > balanced.
> 
> Does not compute, sorry. How would delaying the effect of migrations
> help?
> 
> Suppose we have 2 cpus and 6 tasks. cpu0 has 2 tasks, cpu1 has 4 tasks.
> the group weights are resp. 341 and 682. We compute we have an imbalance
> of 341 and need to migrate 170 to equalize. We achieve this by moving
> the 1 task, such that both cpus end up with 4 tasks.

3 of course.

> After that we want to find weights of 512 and 512. But if we were to
> consider old weights, we'd find 426 and 597 making it appear there is
> still an imbalance. We could end up migrating more, only to later find
> we overshot and now need to go back.
> 
> This is the classical ringing problem.
> 
> I also don't see any up-sides from doing this.
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