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

[Vincent Guittot <vincent.guittot@linaro.org>]

On 3 September 2014 11:11, Preeti U Murthy <preeti@linux.vnet.ibm.com> wrote:
> On 09/01/2014 02:15 PM, Vincent Guittot wrote:
>> On 30 August 2014 19:50, Preeti U Murthy <preeti@linux.vnet.ibm.com> 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.

yes you're right for the test if (rq->nr_running >= 2).

It's not so straight forward for nr_busy_cpus which reflects how many
CPUs have not stopped their tick. The scheduler assumes that the
latter are busy with cfs tasks

>
> 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

I have just replied to your comments on patch 3. Sorry for the delay

> 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.
This patch adds one additional condition that is based on
rq->cfs.h_nr_running so it should not trigger any futile load balance.
Then, I have also take advantage of this patch to clean up
nohz_kick_needed as proposed by Peter but the conditions are the same
than previously (except the one with rq->cfs.h_nr_running)

I can prepare another patchset that will solve the concerns that you
raised for nohz_kick_needed and in patch 3 but i would prefer not
include them in this patchset which is large enough and which subject
is a bit different.
Does it seem ok for you ?

Regards,
Vincent
>
> Regards
> Preeti U Murthy
>