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SubjectRe: [External] Re: [PATCH v2] sched/fair: combine detach into dequeue when migrating task
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Hi,

On 2022/6/10 18:49, Dietmar Eggemann wrote:
> On 09/06/2022 05:53, Chengming Zhou wrote:
>> When we are migrating task out of the CPU, we can combine detach
>> into dequeue_entity() to save the independent detach_entity_cfs_rq()
>> in migrate_task_rq_fair().
>>
>> This optimization is like combining DO_ATTACH in the enqueue_entity()
>> when migrating task to the CPU.
>>
>> So we don't have to traverse the CFS tree twice to do these load
>> detach and propagation.
>
> By `propagation` you refer to the detach_entity_cfs_rq() ->
> propagate_entity_cfs_rq() call?
> This one wouldn't be called anymore with your change.

Yes.

>
> [...]
>
>> @@ -4426,6 +4435,14 @@ static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq);
>> static void
>> dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>> {
>> + int action = UPDATE_TG;
>> +
>> + /*
>> + * If we are migrating task from the CPU, detach load_avg when dequeue.
>> + */
>> + if (entity_is_task(se) && task_of(se)->on_rq == TASK_ON_RQ_MIGRATING)
>
> - if (entity_is_task(se) && task_of(se)->on_rq == TASK_ON_RQ_MIGRATING)
> + if (entity_is_task(se) && task_on_rq_migrating(task_of(se)))

Better, will do.

>
>> + action |= DO_DETACH;
>
> With the `entity_is_task(se)` you make sure we only call
> detach_entity_load_avg() and update_tg_load_avg() for the se
> representing the task itself (and not taskgroups the task might run in).
> So IMHO this looks good.
>
> You save the propagate_entity_cfs_rq(&p->se) call from (2) by doing the
> detach_entity_load_avg(), update_tg_load_avg() for a migrating task
> inside (1) (the task being the first se in the loop )
>
> detach_task()
> deactivate_task()
> dequeue_task_fair()
> for_each_sched_entity(se)
> dequeue_entity()
> update_load_avg() /* (1) */
>
> set_task_cpu()
> migrate_task_rq_fair()
> /* called detach_entity_cfs_rq() before the patch (2) */

Thanks for the description, it's clearer, I should put it in the commit message.

>
> [...]
>
>> @@ -6940,15 +6957,10 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
>> se->vruntime -= min_vruntime;
>> }
>>
>> - if (p->on_rq == TASK_ON_RQ_MIGRATING) {
>> - /*
>> - * In case of TASK_ON_RQ_MIGRATING we in fact hold the 'old'
>> - * rq->lock and can modify state directly.
>> - */
>> - lockdep_assert_rq_held(task_rq(p));
>> - detach_entity_cfs_rq(&p->se);
>> -
>> - } else {
>> + /*
>> + * In case of TASK_ON_RQ_MIGRATING we already detach in dequeue_entity.
>> + */
>> + if (p->on_rq != TASK_ON_RQ_MIGRATING) {
>
> - if (p->on_rq != TASK_ON_RQ_MIGRATING) {
> + if (!task_on_rq_migrating(p)) {

Will do. Thanks!

>
> [...]

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