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Message-ID: <19c6accf-1cc0-489d-9c82-e0ddf982b098@bytedance.com>
Date:   Thu, 16 Nov 2023 13:07:18 +0800
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Subject: Re: [PATCH 1/4] sched/eevdf: Fix vruntime adjustment on reweight
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From:   Abel Wu <wuyun.abel@bytedance.com>
To:     Yiwei Lin <s921975628@gmail.com>
Cc:     Barry Song <21cnbao@gmail.com>,
        Benjamin Segall <bsegall@google.com>,
        Chen Yu <yu.c.chen@intel.com>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        "Gautham R . Shenoy" <gautham.shenoy@amd.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        K Prateek Nayak <kprateek.nayak@amd.com>,
        Mike Galbraith <efault@gmx.de>,
        Qais Yousef <qyousef@layalina.io>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Yicong Yang <yangyicong@huawei.com>,
        Youssef Esmat <youssefesmat@chromium.org>,
        linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Valentin Schneider <valentin.schneider@arm.com>
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 <20231107090510.71322-2-wuyun.abel@bytedance.com>
 <da42f1d7-adfe-485e-987e-3e8dae78b4c2@gmail.com>
 <23cebdf8-1a51-481e-a8c2-e17021099e05@bytedance.com>
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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?