Re: [PATCH v2 1/3] sched/fair: fix rounding issue for asym packing
From: Vincent Guittot
Date: Wed Dec 19 2018 - 10:54:19 EST
On Wed, 19 Dec 2018 at 16:30, Valentin Schneider
> 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,
> 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
ah yes, i thought the nr_running was involved but it's not the case.
This looks better indeed
> and not get any rounding errors.