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

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Date:	Mon, 23 May 2016 12:27:54 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Yuyang Du <yuyang.du@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Wanpeng Li <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 20 May 2016 at 12:12, Morten Rasmussen <morten.rasmussen@....com> wrote:
> 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.

yes,  i haven't found a simple use case yet to demonstrate the
usefulness of this part

>
> 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.