linux-kernel - Re: [PATCH v2] sched/uclamp: Avoid getting unreasonable ucalmp value when rq is idle

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Date:   Fri, 02 Jul 2021 13:12:41 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Qais Yousef <qais.yousef@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Xuewen Yan <xuewen.yan94@...il.com>, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org,
        patrick.bellasi@...bug.net, qperret@...gle.com
Subject: Re: [PATCH v2] sched/uclamp: Avoid getting unreasonable ucalmp value when rq is idle

On 02/07/21 12:54, Qais Yousef wrote:
> sched/uclamp: Ignore max aggregation if rq is idle
>
> When a task wakes up on an idle rq, uclamp_rq_util_with() would max
> aggregate with rq value. But since there is no task enqueued yet, the
> values are stale based on the last task that was running. When the new

Nit: those values are "intentionally stale" for UCLAMP_MAX, per

  e496187da710 ("sched/uclamp: Enforce last task's UCLAMP_MAX")

for UCLAMP_MIN we'll set uclamp_none(UCLAMP_MIN) == 0 upon dequeueing the
last runnable task, which DTRT.

> task actually wakes up and enqueued, then the rq uclamp values should
> reflect that of the newly woken up task effective uclamp values.
>
> This is a problem particularly for uclamp_max because it default to
                    ^^^^^^^^^^^^
Per the above, it's "only" a problem for UCLAMP_MAX.

> 1024. If a task p with uclamp_max = 512 wakes up, then max aggregation
> would ignore the capping that should apply when this task is enqueued,
> which is wrong.
>
> Fix that by ignoring max aggregation if the rq is idle since in that
> case the effective uclamp value of the rq will be the ones of the task
> that will wake up.
>