linux-kernel - Re: [tip:sched/core] sched/fair: Clean up scale confusion

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Message-ID: <20160520101226.GA30663@e105550-lin.cambridge.arm.com>
Date:	Fri, 20 May 2016 11:12:27 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Yuyang Du <yuyang.du@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>, wanpeng.li@...mail.com,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Benjamin Segall <bsegall@...gle.com>
Subject: Re: [tip:sched/core] sched/fair: Clean up scale confusion

On Fri, May 13, 2016 at 09:23:50AM +0200, Vincent Guittot wrote:
> On 12 May 2016 at 21:42, Yuyang Du <yuyang.du@...el.com> wrote:
> > On Thu, May 12, 2016 at 03:31:27AM -0700, tip-bot for Peter Zijlstra wrote:
> >> Commit-ID:  1be0eb2a97d756fb7dd8c9baf372d81fa9699c09
> >> Gitweb:     http://git.kernel.org/tip/1be0eb2a97d756fb7dd8c9baf372d81fa9699c09
> >> Author:     Peter Zijlstra <peterz@...radead.org>
> >> AuthorDate: Fri, 6 May 2016 12:21:23 +0200
> >> Committer:  Ingo Molnar <mingo@...nel.org>
> >> CommitDate: Thu, 12 May 2016 09:55:33 +0200
> >>
> >> sched/fair: Clean up scale confusion
> >>
> >> Wanpeng noted that the scale_load_down() in calculate_imbalance() was
> >> weird. I agree, it should be SCHED_CAPACITY_SCALE, since we're going
> >> to compare against busiest->group_capacity, which is in [capacity]
> >> units.
> 
> In fact, load_above_capacity is only about load and not about capacity.
> 
> load_above_capacity -= busiest->group_capacity is an optimization (may
> be a wronf one) of
> load_above_capacity -= busiest->group_capacity * SCHED_LOAD_SCALE /
> SCHED_CAPACITY_SCALE
> 
> so we subtract load to load

I like your approach as you compute the desired minimum load, which is
essentially finding the number of NICE_0_LOAD task we want in the group,
and then determine how much excess load there is. So it becomes quite
clear that it is load.

While it preserves existing behaviour I would question the whole
NICE_0_LOAD assumption. It totally falls apart with PELT and if we have
tasks with nice != 0.

Also, it doesn't address the existing unit issue as load_above_capacity
is later multiplied by busiest->group_capacity when computing the
imbalance. As said in the other thread, we should either kill the
minimum load estimation that assumes always-running NICE_0_LOAD tasks,
or at least make sure the scaling of load_above_capacity is correct.
Patches attempting either solution are in the other thread.