linux-kernel - Re: [PATCH v2 1/3] sched/fair: fix rounding issue for asym packing

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Date:   Wed, 19 Dec 2018 15:30:06 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [PATCH v2 1/3] sched/fair: fix rounding issue for asym packing

On 19/12/2018 15:20, Vincent Guittot wrote:
[...]
>> Oh yes, I never said it didn't work - I was doing some investigation on
>> the reason as to why we'd need this fix, because it's wasn't explicit from
>> the commit message.
>>
>> The rounding errors are countered by the +1, yes, but I'd rather remove
>> the errors altogether and go for the snippet I suggested in my previous
>> reply.
> 
> except that you don't always want to migrate all group load.
> I prefer keeping current algorithm and fix it for now. Trying "new"
> thing can come in a 2nd step

We already set the imbalance as the whole group load, we just introduce
rounding errors inbetween. As I've already said:

in update_sg_lb_stats() we do:

        sgs->avg_load = (sgs->group_load*SCHED_CAPACITY_SCALE) / sgs->group_capacity;

and in check_asym_packing() we do:

        env->imbalance = DIV_ROUND_CLOSEST(
                sds->busiest_stat.avg_load * sds->busiest_stat.group_capacity,
                SCHED_CAPACITY_SCALE)

So we end up with something like:

                    group_load * SCHED_CAPACITY_SCALE * group_capacity
        imbalance = --------------------------------------------------
                            group_capacity * SCHED_CAPACITY_SCALE

Which we could reduce down to:

        imbalance = group_load

and not get any rounding errors.