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

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Message-ID: <0e522193-b4af-40ad-9762-41bfa643dd60@bytedance.com>
Date: Mon, 22 Jul 2024 17:39:34 +0800
From: Chuyi Zhou <zhouchuyi@...edance.com>
To: Dietmar Eggemann <dietmar.eggemann@....com>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
 rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com
Cc: chengming.zhou@...ux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/fair: Sync se's load_avg with cfs_rq in
 reweight_task

Hello,

在 2024/7/22 14:34, Dietmar Eggemann 写道:
> On 20/07/2024 07:12, 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);
> 
> IIUC, you only want to sync the sleeping task with its cfs_rq. IMHO,
> sync_entity_load_avg() should be used here instead of update_load_avg().
> The latter is doing much more than this.

Indeed, sync_entity_load_avg() is better.

> 
>>   	reweight_entity(cfs_rq, se, lw->weight);
>>   	load->inv_weight = lw->inv_weight;
>>   }
> 
> Maybe even do this in reweight_entity()?. You would have to do it under
> 'if (!se->on_rq) in reweight_task() anyway I assume.
> 

Yes, we can do it reweight_entity() and it's more clear.

Thanks for your suggestion.