Re: [External] Re: [PATCH v2 07/10] sched/fair: use update_load_avg() to attach/detach entity load_avg

[Chengming Zhou <zhouchengming@bytedance.com>]

On 2022/7/15 19:18, Dietmar Eggemann wrote:
> On 13/07/2022 06:04, Chengming Zhou wrote:
>> Since update_load_avg() support DO_ATTACH and DO_DETACH now, we can
>> use update_load_avg() to implement attach/detach entity load_avg.
>>
>> Another advantage of using update_load_avg() is that it will check
>> last_update_time before attach or detach, instead of unconditional
>> attach/detach in the current code.
>>
>> This way can avoid some corner problematic cases of load tracking,
>> like twice attach problem, detach unattached NEW task problem.
> 
> This explanation is somewhat hard to follow for me. Since both issues
> have been fixed already (you mention this further below) you're saying
> that with you change you don't reintroduce them?

Sorry for this not very clear explanation.

Yes, both issues have been fixed already, what I want to say is that bugfix
brings its own problem and limitation mentioned below.

So I want to use another way to solve these problems better.

> 
>> 1. switch to fair class (twice attach problem)
>>
>> p->sched_class = fair_class;  --> p.se->avg.last_update_time = 0
>> if (queued)
>>   enqueue_task(p);
>>     ...
>>       enqueue_entity()
>>         update_load_avg(UPDATE_TG | DO_ATTACH)
>>           if (!se->avg.last_update_time && (flags & DO_ATTACH))  --> true
>>             attach_entity_load_avg()  --> attached, will set last_update_time
>> check_class_changed()
>>   switched_from() (!fair)
>>   switched_to()   (fair)
>>     switched_to_fair()
>>       attach_entity_load_avg()  --> unconditional attach again!
>>
>> 2. change cgroup of NEW task (detach unattached task problem)
>>
>> sched_move_group(p)
>>   if (queued)
>>     dequeue_task()
>>   task_move_group_fair()
>>     detach_task_cfs_rq()
>>       detach_entity_load_avg()  --> detach unattached NEW task
>>     set_task_rq()
>>     attach_task_cfs_rq()
>>       attach_entity_load_avg()
>>   if (queued)
>>     enqueue_task()
>>
>> These problems have been fixed in commit 7dc603c9028e
>> ("sched/fair: Fix PELT integrity for new tasks"), which also
>> bring its own problems.
>>
>> First, it add a new task state TASK_NEW and an unnessary limitation
>> that we would fail when change the cgroup of TASK_NEW tasks.

This is the limitation that bugfix has brought.

We can't change cgroup or switch to fair for task with last_update_time=0
if we don't have conditional detach/attach.

So we have to:

1. !fair task also need to set last_update_time.
2. cpu_cgroup_can_attach() have to wait for TASK_NEW to fully attached.

>>
>> Second, it attach entity load_avg in post_init_entity_util_avg(),
>> in which we only set sched_avg last_update_time for !fair tasks,
>> will cause PELT integrity problem when switched_to_fair().
> 
> I guess those PELT integrity problems are less severe since we have the
> enqueue_task_fair() before the switched_to_fair() for enqueued tasks. So
> we always decay the time the task spend outside fair.

enqueue_task_fair()
  enqueue_entity()
    update_load_avg()
      if (se->avg.last_update_time && !(flags & SKIP_AGE_LOAD))  --> true
        __update_load_avg_se(now, cfs_rq, se);  --> (1)

We can see above, for queued !fair task, (1) will deay the delta time
(now - se.avg.last_update_time) even for a NEW !fair task.

> 
> Looks to me that you want to replace this by your `freeze PELT when
> outside fair` model.

Yes, want to freeze PELT for two !fair cases:

1. !fair task hasn't been fair before: will still have its init load_avg
   when switch to fair.

2. !fair task has been switched_from_fair(): will still keep its lastest
   load_avg when switch to fair.

> 
>> This patch make update_load_avg() the only location of attach/detach,
>> and can handle these corner cases like change cgroup of NEW tasks,
>> by checking last_update_time before attach/detach.
> 
> [...]
> 
>> @@ -11527,9 +11522,7 @@ 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);
>> +	update_load_avg(cfs_rq, se, UPDATE_TG | DO_DETACH);
> 
> IMHO, the DO_[DE|AT]TACH comments in update_load_avg() would have to be
> updated in this case.

Correct, will do.

Thanks.

> 
> [...]