linux-kernel - Re: [PATCH v5 8/9] sched/fair: defer task sched_avg attach to enqueue

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Date:   Thu, 18 Aug 2022 19:03:11 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     vincent.guittot@...aro.org, dietmar.eggemann@....com,
        mingo@...hat.com, rostedt@...dmis.org, bsegall@...gle.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org, tj@...nel.org
Subject: Re: [PATCH v5 8/9] sched/fair: defer task sched_avg attach to
 enqueue_entity()

On 2022/8/18 18:39, Peter Zijlstra wrote:
> On Thu, Aug 18, 2022 at 11:43:42AM +0800, Chengming Zhou wrote:
>> When wake_up_new_task(), we would use post_init_entity_util_avg()
>> to init util_avg/runnable_avg based on cpu's util_avg at that time,
>> then attach task sched_avg to cfs_rq.
>>
>> Since enqueue_entity() would always attach any unattached task entity,
>> so we can defer this work to enqueue_entity().
>>
>> post_init_entity_util_avg(p)
>>   attach_entity_cfs_rq()  --> (1)
>> activate_task(rq, p)
>>   enqueue_task() := enqueue_task_fair()
>>   enqueue_entity()
>>     update_load_avg(cfs_rq, se, UPDATE_TG | DO_ATTACH)
>>       if (!se->avg.last_update_time && (flags & DO_ATTACH))
>>         attach_entity_load_avg()  --> (2)
>>
>> This patch defer attach from (1) to (2)
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
>> ---
>>  kernel/sched/fair.c | 4 ----
>>  1 file changed, 4 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index e0d34ecdabae..aacf38a72714 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -799,8 +799,6 @@ void init_entity_runnable_average(struct sched_entity *se)
>>  	/* when this task enqueue'ed, it will contribute to its cfs_rq's load_avg */
>>  }
>>  
>> -static void attach_entity_cfs_rq(struct sched_entity *se);
>> -
>>  /*
>>   * With new tasks being created, their initial util_avgs are extrapolated
>>   * based on the cfs_rq's current util_avg:
>> @@ -863,8 +861,6 @@ void post_init_entity_util_avg(struct task_struct *p)
>>  		se->avg.last_update_time = cfs_rq_clock_pelt(cfs_rq);
>>  		return;
>>  	}
>> -
>> -	attach_entity_cfs_rq(se);
>>  }
> 
> There are comments with update_cfs_rq_load_avg() and
> remove_entity_load_avg() that seem to rely on post_init_entity_util()
> doing this attach.
> 
> If that is no longer true; at the very least those comments need to be
> updated, but also, I don't immediately see why that's no longer the
> case, so please explain.

This attach in post_init_entity_util() will be done in enqueue_entity()
-> update_load_avg(cfs_rq, se, UPDATE_TG | DO_ATTACH) loop.

So these comments should be updated in the next version.

Thanks!