linux-kernel - Re: [PATCH RFC v2] sched/fair: simplify the work when reweighting entity(Internet mail)

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Message-ID: <4E0D3766-146C-4864-984A-79B90F0B2EEF@tencent.com>
Date:   Tue, 11 Aug 2020 11:19:41 +0000
From:   benbjiang(蒋彪) <benbjiang@...cent.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
CC:     Jiang Biao <benbjiang@...il.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "bsegall@...gle.com" <bsegall@...gle.com>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2] sched/fair: simplify the work when reweighting
 entity(Internet mail)

Hi,

> On Aug 11, 2020, at 5:50 PM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> 
> On 06/08/2020 18:14, Jiang Biao wrote:
>> From: Jiang Biao <benbjiang@...cent.com>
>> 
>> If a se is on_rq when reweighting entity, all we need should be
>> updating the load of cfs_rq, other dequeue/enqueue work could be
>> redundant, such as,
>> * nr_running--/nr_running++
>> 
>> Even though the following dequeue/enqueue path would never be reached
>> * account_numa_dequeue/account_numa_enqueue
>> * list_del/list_add from/into cfs_tasks
>> but it could be a little confusing.
>> 
>> Simplifying the logic could be helpful to reduce a litte overhead for
>> hot path, and make it cleaner and more readable.
> 
> LGTM. I guess you have to resend it w/o the RFC tag.
Will do soon.
> 
> Maybe you can rework the patch header a little?
> 
> Something like this:
> 
> The code in reweight_entity() can be simplified.
> 
> For a sched entity on the rq, the entity accounting can be replaced by
> cfs_rq instantaneous load updates currently called from within the
> entity accounting.
> 
> Even though an entity on the rq can't represent a task in
> reweight_entity() (a task is always dequeued before calling this
> function) and so the numa task accounting and the rq->cfs_tasks list
> management of the entity accounting are never called, the redundant
> cfs_rq->nr_running decrement/increment will be avoided.
That’s a much better. I’ll pick the header if you don’t mind. :)

Thanks a lot for your kindly reply and detailed explanation.

Regards,
Jiang
> 
>> 
>> Signed-off-by: Jiang Biao <benbjiang@...cent.com>
>> ---
>> v2<-v1: 
>> - Amend the commit log.
>> 
>> kernel/sched/fair.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 04fa8dbcfa4d..18a8fc7bd0de 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -3086,7 +3086,7 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>> 		/* commit outstanding execution time */
>> 		if (cfs_rq->curr == se)
>> 			update_curr(cfs_rq);
>> -		account_entity_dequeue(cfs_rq, se);
>> +		update_load_sub(&cfs_rq->load, se->load.weight);
>> 	}
>> 	dequeue_load_avg(cfs_rq, se);
>> 
>> @@ -3102,7 +3102,7 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>> 
>> 	enqueue_load_avg(cfs_rq, se);
>> 	if (se->on_rq)
>> -		account_entity_enqueue(cfs_rq, se);
>> +		update_load_add(&cfs_rq->load, se->load.weight);
>> 
>> }