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SubjectRe: [PATCH v5 8/9] sched/fair: defer task sched_avg attach to enqueue_entity()
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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@bytedance.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!

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