Re: [PATCH v4] sched/fair: Sync se's load_avg with cfs_rq in reweight_task

[Chuyi Zhou <zhouchuyi@bytedance.com>]

Friendly Ping.

在 2024/8/5 21:41, Vincent Guittot 写道:
> On Mon, 29 Jul 2024 at 12:47, Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
>>
>> In reweight_task(), there are two situations:
>>
>> 1. The task was on_rq, then the task's load_avg is accurate because in
>> __sched_setscheduler()/set_user_nice(), we would dequeue the on_rq tasks
>> before doing reweight. The task's load_avg would be synchronized with
>> cfs_rq through update_load_avg() in dequeue_task().
>>
>> 2. The task is sleeping, its load_avg might not have been updated for some
>> time, which can result in inaccurate dequeue_load_avg() in
>> reweight_entity().
>>
>> This patch solves this by using sync_entity_load_avg() to synchronize the
>> load_avg of se with cfs_rq before dequeue_load_avg() in reweight_entity().
>> For tasks were on_rq, since we already update load_avg to accurate values
>> in dequeue_task(), this change will not have other effects due to the short
>> time interval between the two updates.
>>
>> Suggested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
>> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
>> Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
>> Reviewed-by: Vishal Chourasia <vishalc@linux.ibm.com>
>> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> 
> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> 
>>
>> ---
>> Changes in v4:
>> - Fix the 'if else' code style issue.(Dietmar)
>> - Add a description of __sched_setscheduler()/set_user_nice() in the commit
>>    log.(Dietmar)
>> - Add comment before calling sync_entity_load_avg().(Qais)
>> Changes in v3:
>> - use sync_entity_load_avg() rather than update_load_avg() to sync the
>> sleeping task with its cfs_rq suggested by Dietmar.
>> - Link t0 v2: https://lore.kernel.org/lkml/20240720051248.59608-1-zhouchuyi@bytedance.com/
>> Changes in v2:
>> - change the description in commit log.
>> - use update_load_avg() in reweight_task() rather than in reweight_entity
>> suggested by chengming.
>> - Link to v1: https://lore.kernel.org/lkml/20240716150840.23061-1-zhouchuyi@bytedance.com/
>> ---
>>   kernel/sched/fair.c | 46 +++++++++++++++++++++++++++------------------
>>   1 file changed, 28 insertions(+), 18 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 9057584ec06d..1e3c7c582541 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -3669,11 +3669,32 @@ dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>          cfs_rq->avg.load_sum = max_t(u32, cfs_rq->avg.load_sum,
>>                                            cfs_rq->avg.load_avg * PELT_MIN_DIVIDER);
>>   }
>> +
>> +static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
>> +{
>> +       return u64_u32_load_copy(cfs_rq->avg.last_update_time,
>> +                                cfs_rq->last_update_time_copy);
>> +}
>> +
>> +/*
>> + * Synchronize entity load avg of dequeued entity without locking
>> + * the previous rq.
>> + */
>> +static void sync_entity_load_avg(struct sched_entity *se)
>> +{
>> +       struct cfs_rq *cfs_rq = cfs_rq_of(se);
>> +       u64 last_update_time;
>> +
>> +       last_update_time = cfs_rq_last_update_time(cfs_rq);
>> +       __update_load_avg_blocked_se(last_update_time, se);
>> +}
>> +
>>   #else
>>   static inline void
>>   enqueue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { }
>>   static inline void
>>   dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { }
>> +static void sync_entity_load_avg(struct sched_entity *se) { }
>>   #endif
>>
>>   static void reweight_eevdf(struct sched_entity *se, u64 avruntime,
>> @@ -3795,7 +3816,14 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>>                  if (!curr)
>>                          __dequeue_entity(cfs_rq, se);
>>                  update_load_sub(&cfs_rq->load, se->load.weight);
>> +       } else if (entity_is_task(se)) {
>> +               /*
>> +                * If the task is sleeping, we need to synchronize entity load avg
>> +                * before dequeue_load_avg().
>> +                */
>> +               sync_entity_load_avg(se);
>>          }
>> +
>>          dequeue_load_avg(cfs_rq, se);
>>
>>          if (se->on_rq) {
>> @@ -4034,11 +4062,6 @@ static inline bool load_avg_is_decayed(struct sched_avg *sa)
>>          return true;
>>   }
>>
>> -static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
>> -{
>> -       return u64_u32_load_copy(cfs_rq->avg.last_update_time,
>> -                                cfs_rq->last_update_time_copy);
>> -}
>>   #ifdef CONFIG_FAIR_GROUP_SCHED
>>   /*
>>    * Because list_add_leaf_cfs_rq always places a child cfs_rq on the list
>> @@ -4773,19 +4796,6 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
>>          }
>>   }
>>
>> -/*
>> - * Synchronize entity load avg of dequeued entity without locking
>> - * the previous rq.
>> - */
>> -static void sync_entity_load_avg(struct sched_entity *se)
>> -{
>> -       struct cfs_rq *cfs_rq = cfs_rq_of(se);
>> -       u64 last_update_time;
>> -
>> -       last_update_time = cfs_rq_last_update_time(cfs_rq);
>> -       __update_load_avg_blocked_se(last_update_time, se);
>> -}
>> -
>>   /*
>>    * Task first catches up with cfs_rq, and then subtract
>>    * itself from the cfs_rq (task must be off the queue now).
>> --
>> 2.20.1
>>