linux-kernel - Re: [PATCH 3/5] sched/fair: rework load

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Date:   Fri, 19 Jul 2019 16:13:38 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Phil Auld <pauld@...hat.com>
Subject: Re: [PATCH 3/5] sched/fair: rework load_balance

On Fri, 19 Jul 2019 at 15:12, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Jul 19, 2019 at 09:58:23AM +0200, Vincent Guittot wrote:
>
> > @@ -8029,17 +8063,24 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> >               }
> >       }
> >
> > -     /* Adjust by relative CPU capacity of the group */
> > -     sgs->group_capacity = group->sgc->capacity;
> > -     sgs->avg_load = (sgs->group_load*SCHED_CAPACITY_SCALE) / sgs->group_capacity;
> > +     /* Check if dst cpu is idle and preferred to this group */
> > +     if (env->sd->flags & SD_ASYM_PACKING &&
> > +         env->idle != CPU_NOT_IDLE &&
> > +         sgs->sum_h_nr_running &&
> > +         sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu)) {
> > +             sgs->group_asym_capacity = 1;
> > +     }
> >
> > -     if (sgs->sum_h_nr_running)
> > -             sgs->load_per_task = sgs->group_load / sgs->sum_h_nr_running;
> > +     sgs->group_capacity = group->sgc->capacity;
> >
> >       sgs->group_weight = group->group_weight;
> >
> > -     sgs->group_no_capacity = group_is_overloaded(env, sgs);
> > -     sgs->group_type = group_classify(group, sgs);
> > +     sgs->group_type = group_classify(env, group, sgs);
> > +
> > +     /* Computing avg_load makes sense only when group is overloaded */
> > +     if (sgs->group_type != group_overloaded)
>
> The comment seems to suggest you meant: ==

yes looks like you're right :-(

>
> > +             sgs->avg_load = (sgs->group_load*SCHED_CAPACITY_SCALE) /
> > +                             sgs->group_capacity;
> >  }
> >
> >  /**
> > @@ -8070,7 +8111,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> >        */
> >       if (sgs->group_type == group_misfit_task &&
> >           (!group_smaller_max_cpu_capacity(sg, sds->local) ||
> > -          !group_has_capacity(env, &sds->local_stat)))
> > +          sds->local_stat.group_type != group_has_spare))
> >               return false;
> >
> >       if (sgs->group_type > busiest->group_type)
> > @@ -8079,11 +8120,18 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> >       if (sgs->group_type < busiest->group_type)
> >               return false;
> >
> > -     if (sgs->avg_load <= busiest->avg_load)
> > +     /* Select the overloaded group with highest avg_load */
> > +     if (sgs->group_type == group_overloaded &&
> > +         sgs->avg_load <= busiest->avg_load)
>
> And this code does too; because with the above '!=', you're comparing
> uninitialized data here, no?

avg_load is always 0
and the load_balance was quite conservative when system was overloaded

>
> > +             return false;
> > +
> > +     /* Prefer to move from lowest priority CPU's work */
> > +     if (sgs->group_type == group_asym_capacity && sds->busiest &&
> > +         sched_asym_prefer(sg->asym_prefer_cpu, sds->busiest->asym_prefer_cpu))
> >               return false;
> >
> >       if (!(env->sd->flags & SD_ASYM_CPUCAPACITY))
> > -             goto asym_packing;
> > +             goto spare_capacity;
> >
> >       /*
> >        * Candidate sg has no more than one task per CPU and