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

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Date:   Fri, 19 Jul 2019 16:02:15 +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 14:54, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Jul 19, 2019 at 09:58:23AM +0200, Vincent Guittot wrote:
>
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 67f0acd..472959df 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5376,18 +5376,6 @@ static unsigned long capacity_of(int cpu)
> >       return cpu_rq(cpu)->cpu_capacity;
> >  }
> >
> > -static unsigned long cpu_avg_load_per_task(int cpu)
> > -{
> > -     struct rq *rq = cpu_rq(cpu);
> > -     unsigned long nr_running = READ_ONCE(rq->cfs.h_nr_running);
> > -     unsigned long load_avg = cpu_runnable_load(rq);
> > -
> > -     if (nr_running)
> > -             return load_avg / nr_running;
> > -
> > -     return 0;
> > -}
> > -
> >  static void record_wakee(struct task_struct *p)
> >  {
> >       /*
>
> > @@ -7646,7 +7669,6 @@ static unsigned long task_h_load(struct task_struct *p)
> >  struct sg_lb_stats {
> >       unsigned long avg_load; /*Avg load across the CPUs of the group */
> >       unsigned long group_load; /* Total load over the CPUs of the group */
> > -     unsigned long load_per_task;
> >       unsigned long group_capacity;
> >       unsigned long group_util; /* Total utilization of the group */
> >       unsigned int sum_nr_running; /* Nr tasks running in the group */
>
>
> > @@ -8266,76 +8293,6 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> >  }
> >
> >  /**
> > - * fix_small_imbalance - Calculate the minor imbalance that exists
> > - *                   amongst the groups of a sched_domain, during
> > - *                   load balancing.
> > - * @env: The load balancing environment.
> > - * @sds: Statistics of the sched_domain whose imbalance is to be calculated.
> > - */
> > -static inline
> > -void fix_small_imbalance(struct lb_env *env, struct sd_lb_stats *sds)
> > -{
> > -     unsigned long tmp, capa_now = 0, capa_move = 0;
> > -     unsigned int imbn = 2;
> > -     unsigned long scaled_busy_load_per_task;
> > -     struct sg_lb_stats *local, *busiest;
> > -
> > -     local = &sds->local_stat;
> > -     busiest = &sds->busiest_stat;
> > -
> > -     if (!local->sum_h_nr_running)
> > -             local->load_per_task = cpu_avg_load_per_task(env->dst_cpu);
> > -     else if (busiest->load_per_task > local->load_per_task)
> > -             imbn = 1;
> > -
> > -     scaled_busy_load_per_task =
> > -             (busiest->load_per_task * SCHED_CAPACITY_SCALE) /
> > -             busiest->group_capacity;
> > -
> > -     if (busiest->avg_load + scaled_busy_load_per_task >=
> > -         local->avg_load + (scaled_busy_load_per_task * imbn)) {
> > -             env->imbalance = busiest->load_per_task;
> > -             return;
> > -     }
> > -
> > -     /*
> > -      * OK, we don't have enough imbalance to justify moving tasks,
> > -      * however we may be able to increase total CPU capacity used by
> > -      * moving them.
> > -      */
> > -
> > -     capa_now += busiest->group_capacity *
> > -                     min(busiest->load_per_task, busiest->avg_load);
> > -     capa_now += local->group_capacity *
> > -                     min(local->load_per_task, local->avg_load);
> > -     capa_now /= SCHED_CAPACITY_SCALE;
> > -
> > -     /* Amount of load we'd subtract */
> > -     if (busiest->avg_load > scaled_busy_load_per_task) {
> > -             capa_move += busiest->group_capacity *
> > -                         min(busiest->load_per_task,
> > -                             busiest->avg_load - scaled_busy_load_per_task);
> > -     }
> > -
> > -     /* Amount of load we'd add */
> > -     if (busiest->avg_load * busiest->group_capacity <
> > -         busiest->load_per_task * SCHED_CAPACITY_SCALE) {
> > -             tmp = (busiest->avg_load * busiest->group_capacity) /
> > -                   local->group_capacity;
> > -     } else {
> > -             tmp = (busiest->load_per_task * SCHED_CAPACITY_SCALE) /
> > -                   local->group_capacity;
> > -     }
> > -     capa_move += local->group_capacity *
> > -                 min(local->load_per_task, local->avg_load + tmp);
> > -     capa_move /= SCHED_CAPACITY_SCALE;
> > -
> > -     /* Move if we gain throughput */
> > -     if (capa_move > capa_now)
> > -             env->imbalance = busiest->load_per_task;
> > -}
> > -
> > -/**
> >   * calculate_imbalance - Calculate the amount of imbalance present within the
> >   *                    groups of a given sched_domain during load balance.
> >   * @env: load balance environment
>
> Maybe strip this out first, in a separate patch. It's all magic doo-doo.

If I remove that 1st, we will have commit in the branch that might
regress some performance temporarily and bisect might spot it while
looking for a culprit, isn't it ?