linux-kernel - Re: [PATCH] fair/util_est: Separate util_est_dequeue() for cfs_rq_util

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Date:   Fri, 11 Dec 2020 12:30:33 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Xuewen Yan <xuewen.yan94@...il.com>, patrick.bellasi@....com,
        vincent.guittot@...aro.org, peterz@...radead.org
Cc:     mingo@...hat.com, juri.lelli@...hat.com, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        linux-kernel@...r.kernel.org, Xuewen.Yan@...soc.com,
        xuewyan@...mail.com
Subject: Re: [PATCH] fair/util_est: Separate util_est_dequeue() for
 cfs_rq_util_change

Hi Yan,

On 09/12/2020 11:44, Xuewen Yan wrote:
> when a task dequeued, it will update it's util, and cfs_rq_util_change
> would check rq's util, if the cfs_rq->avg.util_est.enqueued is bigger
> than  cfs_rq->avg.util_avg, but because the cfs_rq->avg.util_est.enqueued
> didn't be decreased, this would cause bigger cfs_rq_util by mistake,
> as a result, cfs_rq_util_change may change freq unreasonablely.
> 
> separate the util_est_dequeue() into util_est_dequeue() and
> util_est_update(), and dequeue the _task_util_est(p) before update util.

The issue is that sugov_update_[shared\|single] -> sugov_get_util() ->
cpu_util_cfs() operates on an old  cfs_rq->avg.util_est.enqueued value?

cpu_util_cfs()

    if (sched_feat(UTIL_EST))
        util = max_t(util, READ_ONCE(rq->cfs.avg.util_est.enqueued))
                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

dequeue_task_fair() (w/ your patch, moving (1) before (2))

    /* (1) update cfs_rq->avg.util_est.enqueued */
    util_est_dequeue()

    /* (2) potential p->se.avg.util_avg update */
    /* 2 for loops */
    for_each_sched_entity()

        /* this can only lead to a freq change for a root cfs_rq */
        (dequeue_entity() ->) update_load_avg() -> cfs_rq_util_change()
         -> cpufreq_update_util() ->...-> sugov_update_[shared\|single]

    /* (3) potential update p->se.avg.util_est */
    util_est_update()


We do need (3) after (2) because of:

util_est_update()
    ...
    ue.enqueued = (task_util(p) | UTIL_AVG_UNCHANGED); task_util
    ...           ^^^^^^^^^^^^^
                  p->se.avg.util_avg


Did I get this right?

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