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Date: Sun, 9 Jun 2024 16:08:50 +0800
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Subject: Re: [PATCH] sched: Initialize the vruntime of a new task when it is
 first enqueued
To: Peter Zijlstra <peterz@infradead.org>
CC: <mingo@redhat.com>, <juri.lelli@redhat.com>, <vincent.guittot@linaro.org>,
	<dietmar.eggemann@arm.com>, <rostedt@goodmis.org>, <bsegall@google.com>,
	<mgorman@suse.de>, <bristot@redhat.com>, <vschneid@redhat.com>,
	<linux-kernel@vger.kernel.org>
References: <20240606121133.2218723-1-zhangqiao22@huawei.com>
 <20240607103043.GO8774@noisy.programming.kicks-ass.net>
From: Zhang Qiao <zhangqiao22@huawei.com>
In-Reply-To: <20240607103043.GO8774@noisy.programming.kicks-ass.net>
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在 2024/6/7 18:30, Peter Zijlstra 写道:
> On Thu, Jun 06, 2024 at 08:11:33PM +0800, Zhang Qiao wrote:
>> When create a new task, we initialize vruntime of the new task
>> at sched_cgroup_fork(). However, the timing of executing this
>> action is too early and may not be accurate.
>>
>> Because it use 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. At the same
>> time, place_entity in task_fork_fair() is useless, remove it.
> 
> The better argument would've looked at history to see why the code was
> the way it is and then verify those reasons are no longer valid.
> 
> Specifically, I think these are remains of child_runs_first, and that is
> now gone.> > Can you verify and update accordingly?

Initially, __enqueue_entity() was in task_new_fair(), in order to schedule
according to se->vruntime, only a"se->vruntime = cfs_rq->min_vruntime" was
added here. This modification  was introduced by commit e9acbff648 ("sched: introduce se->vruntime").

Then,the commit 4d78e7b656aa("sched: new task placement for vruntime") added proper
new task placement for the vruntime based math, this also requires the new task's vruntime value.

The commit aeb73b040399("sched: clean up new task placement") clean up code and extract
a place_entity() helper function.

To summarize, the place_entity() in task_fork_fair() was for the child_runs_first and enqueue_entity,
After remove the child_runs_first and enqueue_entity from task_fork_fair(), we can remove this place_entity().


> 
>> Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
>> ---
>>  kernel/sched/core.c |  2 +-
>>  kernel/sched/fair.c | 16 ----------------
>>  2 files changed, 1 insertion(+), 17 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 efce2d36a783..bb5f376fd51e 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -12702,23 +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);
>> -
>>  	set_task_max_allowed_capacity(p);
>> -
>> -	cfs_rq = task_cfs_rq(current);
>> -	curr = cfs_rq->curr;
>> -	if (curr) {
>> -		update_rq_clock(rq);
>> -		update_curr(cfs_rq);
>> -	}
>> -	place_entity(cfs_rq, se, ENQUEUE_INITIAL);
>> -	rq_unlock(rq, &rf);
>>  }
>>  
>>  /*
>> -- 
>> 2.18.0.huawei.25
>>
> .
>