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

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Date:   Fri, 2 Jul 2021 14:00:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Xuewen Yan <xuewen.yan94@...il.com>, valentin.schneider@....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 Fri, Jul 02, 2021 at 12:54:21PM +0100, Qais Yousef wrote:
> Yep. How about the below?
> 
> --->8---
> 
> 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
> 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
> 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.
> 
> --->8---

Much better, I've updated it. Thanks!