Re: [PATCH] sched/fair: Fix ftq noise bench regression

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

On 21 March 2017 at 18:46, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
> Hi Vincent,
>
> On 17/03/17 13:47, Vincent Guittot wrote:
>
> [...]
>
>> Reported-by: ying.huang@linux.intel.com
>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
>> Fixes: 4e5160766fcc ("sched/fair: Propagate asynchrous detach")
>
> I thought I can see a difference by running:
>
>  perf stat --null --repeat 10 -- perf bench sched messaging -g 50 -l
>  5000
>
> on an Ubuntu 16.10 server system.
>
> Number of entries in the rq->leaf_cfs_rq_list per cpu: ~40
>
> Target: Intel i5-3320M (4 logical cpus)
>
> tip/sched/core: 42.119140365 seconds time elapsed ( +-  0.33% )
>
> + patch       : 42.089557108 seconds time elapsed ( +-  0.37% )
>
> Maybe I need a CPU with more 'logical cpus' or a different test case.
> Anyway, couldn't spot any regression.

ok. I haven't been able to reproduce the regression on my platforms too

>
>> ---
>>  kernel/sched/fair.c | 39 ++++++++++++++++++++++++++++++++++++---
>>  1 file changed, 36 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 2805bd7..007df59 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -3173,6 +3173,36 @@ static inline int propagate_entity_load_avg(struct sched_entity *se)
>>       return 1;
>>  }
>>
>> +/*
>> + * Check if we need to update the load and the utilization of a blocked
>> + * group_entity
>> + */
>> +static inline bool skip_blocked_update(struct sched_entity *se)
>> +{
>> +     struct cfs_rq *gcfs_rq = group_cfs_rq(se);
>> +
>> +     /*
>> +      * If sched_entity still have not null load or utilization, we have to
>> +      * decay it.
>> +      */
>> +     if (se->avg.load_avg || se->avg.util_avg)
>> +             return false;
>> +
>> +     /*
>> +      * If there is a pending propagation, we have to update the load and
>> +      * the utilizaion of the sched_entity
>
> nit pick: s/utilizaion/utilization

yes


>
>> +      */
>> +     if (gcfs_rq->propagate_avg)
>> +             return false;
>> +
>> +     /*
>> +      * Other wise, the load and the utilization of the sched_entity is
>> +      * already null and there is no pending propagation so it will be a
>> +      * waste of time to try to decay it.
>> +      */
>> +     return true;
>> +}
>> +
>>  #else /* CONFIG_FAIR_GROUP_SCHED */
>>
>>  static inline void update_tg_load_avg(struct cfs_rq *cfs_rq, int force) {}
>> @@ -6961,6 +6991,8 @@ static void update_blocked_averages(int cpu)
>>        * list_add_leaf_cfs_rq() for details.
>>        */
>>       for_each_leaf_cfs_rq(rq, cfs_rq) {
>> +             struct sched_entity *se;
>> +
>>               /* throttled entities do not contribute to load */
>>               if (throttled_hierarchy(cfs_rq))
>>                       continue;
>> @@ -6968,9 +7000,10 @@ static void update_blocked_averages(int cpu)
>>               if (update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq, true))
>>                       update_tg_load_avg(cfs_rq, 0);
>>
>> -             /* Propagate pending load changes to the parent */
>> -             if (cfs_rq->tg->se[cpu])
>> -                     update_load_avg(cfs_rq->tg->se[cpu], 0);
>> +             /* Propagate pending load changes to the parent if any */
>> +             se = cfs_rq->tg->se[cpu];
>> +             if (se && !skip_blocked_update(se))
>> +                     update_load_avg(se, 0);
>>       }
>>       rq_unlock_irqrestore(rq, &rf);
>>  }
>>
>
> Why not turn skip_blocked_update(se) into blocked_update_needed(cpu, cfs_rq)?
> Saves a couple of patch lines:

Are you sure that we are saving some patch lines ?

I tend to agree on the name and but not on parameters.
IMO, it's easier to understand the purpose of
blocked_update_needed(se) compared to blocked_update_needed(cpu,
cfs_rq)


>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 007df5953d1a..8001eeb4ec18 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3177,30 +3177,34 @@ static inline int propagate_entity_load_avg(struct sched_entity *se)
>   * Check if we need to update the load and the utilization of a blocked
>   * group_entity
>   */
> -static inline bool skip_blocked_update(struct sched_entity *se)
> +static inline bool blocked_update_needed(int cpu, struct cfs_rq *cfs_rq)
>  {
> -       struct cfs_rq *gcfs_rq = group_cfs_rq(se);
> +       struct sched_entity *se = cfs_rq->tg->se[cpu];
> +
> +       /* cfs_rq of a root task_group has no sched_entity counterpart */
> +       if (!se)
> +               return false;
>
>         /*
>          * If sched_entity still have not null load or utilization, we have to
>          * decay it.
>          */
>         if (se->avg.load_avg || se->avg.util_avg)
> -               return false;
> +               return true;
>
>         /*
>          * If there is a pending propagation, we have to update the load and
>          * the utilizaion of the sched_entity
>          */
> -       if (gcfs_rq->propagate_avg)
> -               return false;
> +       if (cfs_rq->propagate_avg)
> +               return true;
>
>         /*
>          * Other wise, the load and the utilization of the sched_entity is
>          * already null and there is no pending propagation so it will be a
>          * waste of time to try to decay it.
>          */
> -       return true;
> +       return false;
>  }
>
>  #else /* CONFIG_FAIR_GROUP_SCHED */
> @@ -6991,8 +6995,6 @@ static void update_blocked_averages(int cpu)
>          * list_add_leaf_cfs_rq() for details.
>          */
>         for_each_leaf_cfs_rq(rq, cfs_rq) {
> -               struct sched_entity *se;
> -
>                 /* throttled entities do not contribute to load */
>                 if (throttled_hierarchy(cfs_rq))
>                         continue;
> @@ -7001,9 +7003,8 @@ static void update_blocked_averages(int cpu)
>                         update_tg_load_avg(cfs_rq, 0);
>
>                 /* Propagate pending load changes to the parent if any */
> -               se = cfs_rq->tg->se[cpu];
> -               if (se && !skip_blocked_update(se))
> -                       update_load_avg(se, 0);
> +               if (blocked_update_needed(cpu, cfs_rq))
> +                       update_load_avg(cfs_rq->tg->se[cpu], 0);
>         }
>         rq_unlock_irqrestore(rq, &rf);
>  }
>
>
>
>