linux-kernel - Re: [PATCH 1/4] sched/eevdf: Fix vruntime adjustment on reweight

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Date:   Thu, 16 Nov 2023 13:07:18 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Yiwei Lin <s921975628@...il.com>
Cc:     Barry Song <21cnbao@...il.com>,
        Benjamin Segall <bsegall@...gle.com>,
        Chen Yu <yu.c.chen@...el.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        Joel Fernandes <joel@...lfernandes.org>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Mike Galbraith <efault@....de>,
        Qais Yousef <qyousef@...alina.io>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Yicong Yang <yangyicong@...wei.com>,
        Youssef Esmat <youssefesmat@...omium.org>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH 1/4] sched/eevdf: Fix vruntime adjustment on reweight

On 11/16/23 12:48 PM, Abel Wu Wrote:
> On 11/15/23 11:36 PM, Yiwei Lin Wrote:
>>
>>> @@ -3712,8 +3811,17 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>>>       enqueue_load_avg(cfs_rq, se);
>>>       if (se->on_rq) {
>>>           update_load_add(&cfs_rq->load, se->load.weight);
>>> -        if (cfs_rq->curr != se)
>>> -            avg_vruntime_add(cfs_rq, se);
>>> +        if (!curr) {
>>> +            /*
>>> +             * The entity's vruntime has been adjusted, so let's check
>>> +             * whether the rq-wide min_vruntime needs updated too. Since
>>> +             * the calculations above require stable min_vruntime rather
>>> +             * than up-to-date one, we do the update at the end of the
>>> +             * reweight process.
>>> +             */
>>> +            __enqueue_entity(cfs_rq, se);
>>> +            update_min_vruntime(cfs_rq);
>>> +        }
>>>       }
>>>   }
>> Sorry if I am asking stupid question...... It looks like reweight_entity() may have chance to change the weight of cfs_rq->curr entity, but we'll never update_min_vruntime() when reweighting it. Is there any reason that we can skip the update_min_vruntime() for this case?
> 
> No, you are right!

I was intended to update_min_vruntime() if se->on_rq and no matter
it is curr or not, just as you suggested. But after a second thought
I wonder if it is necessary to update *NOW*, since we will always
update_curr() before making any change to cfs_rq. Thoughts?