linux-kernel - Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity

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Date:	Tue, 8 Sep 2015 15:39:37 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steve Muckle <steve.muckle@...aro.org>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"yuyang.du@...el.com" <yuyang.du@...el.com>,
	"mturquette@...libre.com" <mturquette@...libre.com>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	Juri Lelli <Juri.Lelli@....com>,
	"sgurrappadi@...dia.com" <sgurrappadi@...dia.com>,
	"pang.xunlei@....com.cn" <pang.xunlei@....com.cn>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity_orig

On 8 September 2015 at 14:26, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Sep 08, 2015 at 09:22:05AM +0200, Vincent Guittot wrote:
>> No, but
>> sa->util_avg = (sa->util_sum << SCHED_CAPACITY_SHIFT) / LOAD_AVG_MAX;
>> will fix the unit issue.
>
> Tricky that, LOAD_AVG_MAX very much relies on the unit being 1<<10.
>
> And where load_sum already gets a factor 1024 from the weight
> multiplication, util_sum does not get such a factor, and all the scaling
> we do on it loose bits.

fair point

>
> So at the moment we go compute the util_avg value, we need to inflate
> util_sum with an extra factor 1024 in order to make it work.
>
> And seeing that we do the shift up on sa->util_sum without consideration
> of overflow, would it not make sense to add that factor before the
> scaling and into the addition?

Yes this should save 1 left shift and 1 right shift

 >>

>
> Now, given all that, units are a complete mess here, and I'd not mind
> something like:
>
> #if (SCHED_LOAD_SHIFT - SCHED_LOAD_RESOLUTION) != SCHED_CAPACITY_SHIFT
> #error "something usefull"
> #endif

In this case why not simply doing
#define SCHED_CAPACITY_SHIFT SCHED_LOAD_SHIFT
or the opposite ?

>
> somewhere near here.
>
>
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