linux-kernel - Re: [PATCH v2] sched: Initialize the vruntime of a new task when it is first enqueued

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Message-ID: <e1297077-8ab3-4bc7-a713-c0d81eda5d97@huawei.com>
Date: Tue, 16 Jul 2024 13:32:55 +0800
From: "Zhangqiao (2012 lab)" <zhangqiao22@...wei.com>
To: <mingo@...hat.com>, <peterz@...radead.org>, <juri.lelli@...hat.com>,
	<vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
	<rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
	<bristot@...hat.com>, <vschneid@...hat.com>, <Markus.Elfring@....de>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sched: Initialize the vruntime of a new task when it
 is first enqueued

Ping 🙂



在 2024/6/27 21:33, Zhang Qiao 写道:
> When creating a new task, we initialize vruntime of the newly task at
> sched_cgroup_fork(). However, the timing of executing this action is too
> early and may not be accurate.
>
> Because it uses current CPU to init the vruntime, but the new task
> actually runs on the cpu which be assigned at wake_up_new_task().
>
> To optimize this case, we pass ENQUEUE_INITIAL flag to activate_task()
> in wake_up_new_task(), in this way, when place_entity is called in
> enqueue_entity(), the vruntime of the new task will be initialized.
>
> In addition, place_entity() in task_fork_fair() was introduced for two
> reasons:
> 1. Previously, the __enqueue_entity() was in task_new_fair(),
> in order to provide vruntime for enqueueing the newly task, the
> vruntime assignment equation "se->vruntime = cfs_rq->min_vruntime" was
> introduced by commit e9acbff6484d ("sched: introduce se->vruntime").
> This is the initial state of place_entity().
>
> 2. commit 4d78e7b656aa ("sched: new task placement for vruntime") added
> child_runs_first task placement feature which based on vruntime, this
> also requires the new task's vruntime value.
>
> After removing the child_runs_first and enqueue_entity() from
> task_fork_fair(), this place_entity() no longer makes sense, so remove
> it also.
>
> Signed-off-by: Zhang Qiao <zhangqiao22@...wei.com>
> ---
> v2:
> Improve comments and commit log.
>
> v1: https://lore.kernel.org/lkml/20240606121133.2218723-1-zhangqiao22@huawei.com/
> ---
>   kernel/sched/core.c |  2 +-
>   kernel/sched/fair.c | 15 ---------------
>   2 files changed, 1 insertion(+), 16 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bcf2c4cc0522..b4ff595a2dc8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4897,7 +4897,7 @@ void wake_up_new_task(struct task_struct *p)
>   	update_rq_clock(rq);
>   	post_init_entity_util_avg(p);
>   
> -	activate_task(rq, p, ENQUEUE_NOCLOCK);
> +	activate_task(rq, p, ENQUEUE_NOCLOCK | ENQUEUE_INITIAL);
>   	trace_sched_wakeup_new(p);
>   	wakeup_preempt(rq, p, WF_FORK);
>   #ifdef CONFIG_SMP
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8a5b1ae0aa55..bb5f376fd51e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12702,22 +12702,7 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
>    */
>   static void task_fork_fair(struct task_struct *p)
>   {
> -	struct sched_entity *se = &p->se, *curr;
> -	struct cfs_rq *cfs_rq;
> -	struct rq *rq = this_rq();
> -	struct rq_flags rf;
> -
> -	rq_lock(rq, &rf);
> -	update_rq_clock(rq);
> -
>   	set_task_max_allowed_capacity(p);
> -
> -	cfs_rq = task_cfs_rq(current);
> -	curr = cfs_rq->curr;
> -	if (curr)
> -		update_curr(cfs_rq);
> -	place_entity(cfs_rq, se, ENQUEUE_INITIAL);
> -	rq_unlock(rq, &rf);
>   }
>   
>   /*