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

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Message-ID: <CAKfTPtBGn_N7Ox+=D=cBMF6Vo=0qMWhA6Qio5ZYYYXP2Wz1uHw@mail.gmail.com>
Date:	Fri, 13 May 2016 09:23:50 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	Morten Rasmussen <morten.rasmussen@....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 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

>>
>> Reported-by: Wanpeng Li <wanpeng.li@...mail.com>
>> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
>> Cc: Mike Galbraith <efault@....de>
>> Cc: Morten Rasmussen <morten.rasmussen@....com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Yuyang Du <yuyang.du@...el.com>
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Ingo Molnar <mingo@...nel.org>
>
> It is good that this issue is addressed and patch merged, however, for the
> record, Vincent has already had a solution for this, and we had a patch,
> including other cleanups (the latest version is: https://lkml.org/lkml/2016/5/3/925).
> And I think Ben first pointed this out (and we then attempted to address it)
> as far as I can tell.