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

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Message-ID: <20141009151820.GX10832@worktop.programming.kicks-ass.net>
Date:	Thu, 9 Oct 2014 17:18:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.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 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?
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