linux-kernel - Re: [PATCH v7 6/7] sched: replace capacity

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Date:	Fri, 10 Oct 2014 09:17:22 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Rik van Riel <riel@...hat.com>,
	Mike Galbraith <efault@....de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Paul Turner <pjt@...gle.com>,
	Benjamin Segall <bsegall@...gle.com>
Subject: Re: [PATCH v7 6/7] sched: replace capacity_factor by usage

On 9 October 2014 17:18, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Oct 09, 2014 at 04:18:02PM +0200, Vincent Guittot wrote:
>> On 9 October 2014 14:16, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Tue, Oct 07, 2014 at 02:13:36PM +0200, Vincent Guittot wrote:
>> >> +static inline bool
>> >> +group_has_capacity(struct lb_env *env, struct sg_lb_stats *sgs)
>> >>  {
>> >> +     if ((sgs->group_capacity * 100) >
>> >> +                     (sgs->group_usage * env->sd->imbalance_pct))
>> >> +             return true;
>> >
>> > Why the imb_pct there? We're looking for 100% utilization, not 130 or
>> > whatnot, right?
>>
>> Having exactly 100% is quite difficult because of various rounding.
>> So i have added a margin/threshold to prevent any excessive change of the state.
>> I have just to use the same margin/threshold than in other place in
>> load balance.
>>
>
> Yet you failed to mention this anywhere. Also does it really matter?

yes i think it latter because it give a more stable view of the
"overload state" and "have  free capacity state" of the CPU.
One additional point is that the imbalance_pct will ensure that a
cpu/group will not been seen as having capacity if its available
capacity is only 1-5% which will generate spurious task migration

I will add these details in the commit log and in a comment in the code
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