Re: [PATCH v5 08/12] sched: move cfs task on a CPU with higher capacity

[Preeti U Murthy]

On 09/01/2014 02:15 PM, Vincent Guittot wrote:
> On 30 August 2014 19:50, Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx> wrote:
>> Hi Vincent,
>>> index 18db43e..60ae1ce 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -6049,6 +6049,14 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>>>                       return true;
>>>       }
>>>
>>> +     /*
>>> +      * The group capacity is reduced probably because of activity from other
>>> +      * sched class or interrupts which use part of the available capacity
>>> +      */
>>> +     if ((sg->sgc->capacity_orig * 100) > (sgs->group_capacity *
>>> +                             env->sd->imbalance_pct))
>>
>> Wouldn't the check on avg_load let us know if we are packing more tasks
>> in this group than its capacity ? Isn't that the metric we are more
>> interested in?
> 
> With  this patch, we don't want to pack but we prefer to spread the
> task on another CPU than the one which handles the interruption if
> latter uses a significant part of the CPU capacity.
> 
>>
>>> +             return true;
>>> +
>>>       return false;
>>>  }
>>>
>>> @@ -6534,13 +6542,23 @@ static int need_active_balance(struct lb_env *env)
>>>       struct sched_domain *sd = env->sd;
>>>
>>>       if (env->idle == CPU_NEWLY_IDLE) {
>>> +             int src_cpu = env->src_cpu;
>>>
>>>               /*
>>>                * ASYM_PACKING needs to force migrate tasks from busy but
>>>                * higher numbered CPUs in order to pack all tasks in the
>>>                * lowest numbered CPUs.
>>>                */
>>> -             if ((sd->flags & SD_ASYM_PACKING) && env->src_cpu > env->dst_cpu)
>>> +             if ((sd->flags & SD_ASYM_PACKING) && src_cpu > env->dst_cpu)
>>> +                     return 1;
>>> +
>>> +             /*
>>> +              * If the CPUs share their cache and the src_cpu's capacity is
>>> +              * reduced because of other sched_class or IRQs, we trig an
>>> +              * active balance to move the task
>>> +              */
>>> +             if ((capacity_orig_of(src_cpu) * 100) > (capacity_of(src_cpu) *
>>> +                             sd->imbalance_pct))
>>>                       return 1;
>>>       }
>>>
>>> @@ -6643,6 +6661,8 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>>>
>>>       schedstat_add(sd, lb_imbalance[idle], env.imbalance);
>>>
>>> +     env.src_cpu = busiest->cpu;
>>> +
>>>       ld_moved = 0;
>>>       if (busiest->nr_running > 1) {
>>>               /*
>>> @@ -6652,7 +6672,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>>>                * correctly treated as an imbalance.
>>>                */
>>>               env.flags |= LBF_ALL_PINNED;
>>> -             env.src_cpu   = busiest->cpu;
>>>               env.src_rq    = busiest;
>>>               env.loop_max  = min(sysctl_sched_nr_migrate, busiest->nr_running);
>>>
>>> @@ -7359,10 +7378,12 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>>>
>>>  /*
>>>   * Current heuristic for kicking the idle load balancer in the presence
>>> - * of an idle cpu is the system.
>>> + * of an idle cpu in the system.
>>>   *   - This rq has more than one task.
>>> - *   - At any scheduler domain level, this cpu's scheduler group has multiple
>>> - *     busy cpu's exceeding the group's capacity.
>>> + *   - This rq has at least one CFS task and the capacity of the CPU is
>>> + *     significantly reduced because of RT tasks or IRQs.
>>> + *   - At parent of LLC scheduler domain level, this cpu's scheduler group has
>>> + *     multiple busy cpu.
>>>   *   - For SD_ASYM_PACKING, if the lower numbered cpu's in the scheduler
>>>   *     domain span are idle.
>>>   */
>>> @@ -7372,9 +7393,10 @@ static inline int nohz_kick_needed(struct rq *rq)
>>>       struct sched_domain *sd;
>>>       struct sched_group_capacity *sgc;
>>>       int nr_busy, cpu = rq->cpu;
>>> +     bool kick = false;
>>>
>>>       if (unlikely(rq->idle_balance))
>>> -             return 0;
>>> +             return false;
>>>
>>>         /*
>>>       * We may be recently in ticked or tickless idle mode. At the first
>>> @@ -7388,38 +7410,45 @@ static inline int nohz_kick_needed(struct rq *rq)
>>>        * balancing.
>>>        */
>>>       if (likely(!atomic_read(&nohz.nr_cpus)))
>>> -             return 0;
>>> +             return false;
>>>
>>>       if (time_before(now, nohz.next_balance))
>>> -             return 0;
>>> +             return false;
>>>
>>>       if (rq->nr_running >= 2)
>>
>> Will this check ^^ not catch those cases which this patch is targeting?
> 
> This patch is not about how many tasks are running but if the capacity
> of the CPU is reduced because of side activity like interruptions
> which are only visible in the capacity value (with IRQ_TIME_ACCOUNTING
> config) but not in nr_running.
> Even if the capacity is reduced because of RT tasks, nothing ensures
> that the RT task will be running when the tick fires
> 
> Regards,
> Vincent
>>
>> Regards
>> Preeti U Murthy
>>
>>> -             goto need_kick;
>>> +             return true;
>>>
>>>       rcu_read_lock();
>>>       sd = rcu_dereference(per_cpu(sd_busy, cpu));
>>> -
>>>       if (sd) {
>>>               sgc = sd->groups->sgc;
>>>               nr_busy = atomic_read(&sgc->nr_busy_cpus);
>>>
>>> -             if (nr_busy > 1)
>>> -                     goto need_kick_unlock;
>>> +             if (nr_busy > 1) {
>>> +                     kick = true;
>>> +                     goto unlock;
>>> +             }
>>> +
>>>       }
>>>
>>> -     sd = rcu_dereference(per_cpu(sd_asym, cpu));
>>> +     sd = rcu_dereference(rq->sd);
>>> +     if (sd) {
>>> +             if ((rq->cfs.h_nr_running >= 1) &&
>>> +                             ((rq->cpu_capacity * sd->imbalance_pct) <
>>> +                             (rq->cpu_capacity_orig * 100))) {

Ok I understand your explanation above. But I was wondering if you would
need to add this check around rq->cfs.h_nr_running >= 1 in the above two
cases as well.

I have actually raised this concern over whether we should be using
rq->nr_running or cfs_rq->nr_running while we do load balancing in reply
to your patch3. While all our load measurements are about the cfs_rq
load, we use rq->nr_running, which may include tasks from other sched
classes as well. We divide them to get average load per task during load
balancing which is wrong, isn't it? Similarly during nohz_kick_needed(),
we trigger load balancing based on rq->nr_running again.

In this patch too, even if you know that the cpu is being dominated by
tasks that do not belong to cfs class, you would be triggering a futile
load balance if there are no fair tasks to move.

Regards
Preeti U Murthy

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