linux-kernel - Re: [PATCH v2] sched/fair: Sync se's load_avg with cfs_rq in reweight

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Message-ID: <a98d66a7-2513-424e-a4c0-84980b9dbde8@amd.com>
Date: Mon, 22 Jul 2024 13:45:13 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Chengming Zhou <chengming.zhou@...ux.dev>, Chuyi Zhou
	<zhouchuyi@...edance.com>
CC: <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>,
	<vschneid@...hat.com>, <linux-kernel@...r.kernel.org>, Qais Yousef
	<qyousef@...alina.io>
Subject: Re: [PATCH v2] sched/fair: Sync se's load_avg with cfs_rq in
 reweight_task

Hello Chengming,

On 7/22/2024 12:10 PM, Chengming Zhou wrote:
> On 2024/7/22 13:17, K Prateek Nayak wrote:
>> (+ Qais)
>>
>> Hello Chuyi,
>>
>> On 7/20/2024 10:42 AM, Chuyi Zhou wrote:
>>> In reweight_task(), there are two situations:
>>>
>>> 1. The task was on_rq, then the task's load_avg is accurate because we
>>> synchronized it with cfs_rq through update_load_avg() in dequeue_task().
>>>
>>> 2. The task is sleeping, its load_avg might not have been updated for some
>>> time, which can result in inaccurate dequeue_load_avg() in
>>> reweight_entity().
>>>
>>> This patch solves this by using update_load_avg() to synchronize the
>>> load_avg of se with cfs_rq. For tasks were on_rq, since we already update
>>> load_avg to accurate values in dequeue_task(), this change will not have
>>> other effects due to the short time interval between the two updates.
>>>
>>> Signed-off-by: Chuyi Zhou <zhouchuyi@...edance.com>
>>> ---
>>> Changes in v2:
>>> - change the description in commit log.
>>> - use update_load_avg() in reweight_task() rather than in reweight_entity
>>> suggested by chengming.
>>> - Link to v1: https://lore.kernel.org/lkml/20240716150840.23061-1-zhouchuyi@bytedance.com/
>>> ---
>>>   kernel/sched/fair.c | 3 +++
>>>   1 file changed, 3 insertions(+)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index 9057584ec06d..b1e07ce90284 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -3835,12 +3835,15 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>>>       }
>>>   }
>>> +static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags);
>>> +
>>>   void reweight_task(struct task_struct *p, const struct load_weight *lw)
>>>   {
>>>       struct sched_entity *se = &p->se;
>>>       struct cfs_rq *cfs_rq = cfs_rq_of(se);
>>>       struct load_weight *load = &se->load;
>>> +    update_load_avg(cfs_rq, se, 0);
>>
>> Seems to be necessary when we reach here from __setscheduler_params() or
>> set_user_nice() for a sleeping task. Please feel free to add:
>>
>> Reviewed-by: K Prateek Nayak <kprateek.nayak@....com>
>>
>> But since we are on the subject of accurate PELT accounting, one question
>> I have here is whether a reweight_task() for a sleeping task race with
>> its wakeup? Something like the following scenario:
> 
> These two paths are impossible to race, since we have the task->pi_lock.

Ah! Yes, you are right. Thank you for the clarification :)

-- 
Thanks and Regards,
Prateek

> 
> Thanks.
> 
>>
>> [..snip..]
>>