Re: [PATCH v4 6/9] sched/fair: fix another detach on unattached task corner case

[Chengming Zhou <zhouchengming@bytedance.com>]

On 2022/8/17 23:01, Vincent Guittot wrote:
> On Mon, 8 Aug 2022 at 14:58, Chengming Zhou <zhouchengming@bytedance.com> wrote:
>>
>> commit 7dc603c9028e ("sched/fair: Fix PELT integrity for new tasks")
>> fixed two load tracking problems for new task, including detach on
>> unattached new task problem.
>>
>> There still left another detach on unattached task problem for the task
>> which has been woken up by try_to_wake_up() and waiting for actually
>> being woken up by sched_ttwu_pending().
>>
>> try_to_wake_up(p)
>>   cpu = select_task_rq(p)
>>   if (task_cpu(p) != cpu)
>>     set_task_cpu(p, cpu)
>>       migrate_task_rq_fair()
>>         remove_entity_load_avg()       --> unattached
>>         se->avg.last_update_time = 0;
>>       __set_task_cpu()
>>   ttwu_queue(p, cpu)
>>     ttwu_queue_wakelist()
>>       __ttwu_queue_wakelist()
>>
>> task_change_group_fair()
>>   detach_task_cfs_rq()
>>     detach_entity_cfs_rq()
>>       detach_entity_load_avg()   --> detach on unattached task
>>   set_task_rq()
>>   attach_task_cfs_rq()
>>     attach_entity_cfs_rq()
>>       attach_entity_load_avg()
>>
>> The reason of this problem is similar, we should check in detach_entity_cfs_rq()
>> that se->avg.last_update_time != 0, before do detach_entity_load_avg().
>>
>> This patch move detach/attach_entity_cfs_rq() functions up to be together
>> with other load tracking functions to avoid to use another #ifdef CONFIG_SMP.
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
>> ---
>>  kernel/sched/fair.c | 132 +++++++++++++++++++++++---------------------
>>  1 file changed, 68 insertions(+), 64 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index f52e7dc7f22d..4bc76d95a99d 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -874,9 +874,6 @@ void init_entity_runnable_average(struct sched_entity *se)
>>  void post_init_entity_util_avg(struct task_struct *p)
>>  {
>>  }
>> -static void update_tg_load_avg(struct cfs_rq *cfs_rq)
>> -{
>> -}
>>  #endif /* CONFIG_SMP */
>>
>>  /*
>> @@ -3176,6 +3173,7 @@ void reweight_task(struct task_struct *p, int prio)
>>         load->inv_weight = sched_prio_to_wmult[prio];
>>  }
>>
>> +static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq);
>>  static inline int throttled_hierarchy(struct cfs_rq *cfs_rq);
>>
>>  #ifdef CONFIG_FAIR_GROUP_SCHED
>> @@ -4086,6 +4084,71 @@ static void remove_entity_load_avg(struct sched_entity *se)
>>         raw_spin_unlock_irqrestore(&cfs_rq->removed.lock, flags);
>>  }
>>
>> +#ifdef CONFIG_FAIR_GROUP_SCHED
>> +/*
>> + * Propagate the changes of the sched_entity across the tg tree to make it
>> + * visible to the root
>> + */
>> +static void propagate_entity_cfs_rq(struct sched_entity *se)
>> +{
>> +       struct cfs_rq *cfs_rq = cfs_rq_of(se);
>> +
>> +       if (cfs_rq_throttled(cfs_rq))
>> +               return;
>> +
>> +       if (!throttled_hierarchy(cfs_rq))
>> +               list_add_leaf_cfs_rq(cfs_rq);
>> +
>> +       /* Start to propagate at parent */
>> +       se = se->parent;
>> +
>> +       for_each_sched_entity(se) {
>> +               cfs_rq = cfs_rq_of(se);
>> +
>> +               update_load_avg(cfs_rq, se, UPDATE_TG);
>> +
>> +               if (cfs_rq_throttled(cfs_rq))
>> +                       break;
>> +
>> +               if (!throttled_hierarchy(cfs_rq))
>> +                       list_add_leaf_cfs_rq(cfs_rq);
>> +       }
>> +}
>> +#else
>> +static void propagate_entity_cfs_rq(struct sched_entity *se) { }
>> +#endif
>> +
>> +static void detach_entity_cfs_rq(struct sched_entity *se)
>> +{
>> +       struct cfs_rq *cfs_rq = cfs_rq_of(se);
>> +
>> +       /*
>> +        * In case the task sched_avg hasn't been attached:
>> +        * - A forked task which hasn't been woken up by wake_up_new_task().
>> +        * - A task which has been woken up by try_to_wake_up() but is
>> +        *   waiting for actually being woken up by sched_ttwu_pending().
>> +        */
>> +       if (!se->avg.last_update_time)
>> +               return;
> 
> The 2 lines above and the associated comment are the only relevant
> part of the patch, aren't they ?
> Is everything else just code moving from one place to another one
> without change ?

Yes, everything else is just code movement.

Thanks!

> 
>> +
>> +       /* Catch up with the cfs_rq and remove our load when we leave */
>> +       update_load_avg(cfs_rq, se, 0);
>> +       detach_entity_load_avg(cfs_rq, se);
>> +       update_tg_load_avg(cfs_rq);
>> +       propagate_entity_cfs_rq(se);
>> +}
>> +
>> +static void attach_entity_cfs_rq(struct sched_entity *se)
>> +{
>> +       struct cfs_rq *cfs_rq = cfs_rq_of(se);
>> +
>> +       /* Synchronize entity with its cfs_rq */
>> +       update_load_avg(cfs_rq, se, sched_feat(ATTACH_AGE_LOAD) ? 0 : SKIP_AGE_LOAD);
>> +       attach_entity_load_avg(cfs_rq, se);
>> +       update_tg_load_avg(cfs_rq);
>> +       propagate_entity_cfs_rq(se);
>> +}
>> +
>>  static inline unsigned long cfs_rq_runnable_avg(struct cfs_rq *cfs_rq)
>>  {
>>         return cfs_rq->avg.runnable_avg;
>> @@ -4308,11 +4371,8 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
>>  }
>>
>>  static inline void remove_entity_load_avg(struct sched_entity *se) {}
>> -
>> -static inline void
>> -attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) {}
>> -static inline void
>> -detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) {}
>> +static inline void detach_entity_cfs_rq(struct sched_entity *se) {}
>> +static inline void attach_entity_cfs_rq(struct sched_entity *se) {}
>>
>>  static inline int newidle_balance(struct rq *rq, struct rq_flags *rf)
>>  {
>> @@ -11519,62 +11579,6 @@ static inline bool vruntime_normalized(struct task_struct *p)
>>         return false;
>>  }
>>
>> -#ifdef CONFIG_FAIR_GROUP_SCHED
>> -/*
>> - * Propagate the changes of the sched_entity across the tg tree to make it
>> - * visible to the root
>> - */
>> -static void propagate_entity_cfs_rq(struct sched_entity *se)
>> -{
>> -       struct cfs_rq *cfs_rq = cfs_rq_of(se);
>> -
>> -       if (cfs_rq_throttled(cfs_rq))
>> -               return;
>> -
>> -       if (!throttled_hierarchy(cfs_rq))
>> -               list_add_leaf_cfs_rq(cfs_rq);
>> -
>> -       /* Start to propagate at parent */
>> -       se = se->parent;
>> -
>> -       for_each_sched_entity(se) {
>> -               cfs_rq = cfs_rq_of(se);
>> -
>> -               update_load_avg(cfs_rq, se, UPDATE_TG);
>> -
>> -               if (cfs_rq_throttled(cfs_rq))
>> -                       break;
>> -
>> -               if (!throttled_hierarchy(cfs_rq))
>> -                       list_add_leaf_cfs_rq(cfs_rq);
>> -       }
>> -}
>> -#else
>> -static void propagate_entity_cfs_rq(struct sched_entity *se) { }
>> -#endif
>> -
>> -static void detach_entity_cfs_rq(struct sched_entity *se)
>> -{
>> -       struct cfs_rq *cfs_rq = cfs_rq_of(se);
>> -
>> -       /* Catch up with the cfs_rq and remove our load when we leave */
>> -       update_load_avg(cfs_rq, se, 0);
>> -       detach_entity_load_avg(cfs_rq, se);
>> -       update_tg_load_avg(cfs_rq);
>> -       propagate_entity_cfs_rq(se);
>> -}
>> -
>> -static void attach_entity_cfs_rq(struct sched_entity *se)
>> -{
>> -       struct cfs_rq *cfs_rq = cfs_rq_of(se);
>> -
>> -       /* Synchronize entity with its cfs_rq */
>> -       update_load_avg(cfs_rq, se, sched_feat(ATTACH_AGE_LOAD) ? 0 : SKIP_AGE_LOAD);
>> -       attach_entity_load_avg(cfs_rq, se);
>> -       update_tg_load_avg(cfs_rq);
>> -       propagate_entity_cfs_rq(se);
>> -}
>> -
>>  static void detach_task_cfs_rq(struct task_struct *p)
>>  {
>>         struct sched_entity *se = &p->se;
>> --
>> 2.36.1
>>