linux-kernel - Re: [PATCH] sched: Fix UCLAMP_FLAG

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Message-ID: <YMDSEHB5+zlajb4x@google.com>
Date:   Wed, 9 Jun 2021 14:37:04 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, qais.yousef@....com, rickyiu@...gle.com,
        wvw@...gle.com, patrick.bellasi@...bug.net
Cc:     linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH] sched: Fix UCLAMP_FLAG_IDLE setting

On Wednesday 09 Jun 2021 at 14:33:39 (+0000), Quentin Perret wrote:
> The UCLAMP_FLAG_IDLE flag is set on a runqueue when dequeueing the last
> active task to maintain the last uclamp.max and prevent blocked util
> from suddenly becoming visible.
> 
> However, there is an asymmetry in how the flag is set and cleared which
> can lead to having the flag set whilst there are active task on the rq.
> Specifically, the flag is set in the uclamp_rq_inc() path, which is
> called at enqueue time, but cleared in the uclamp_rq_dec_id() which is
> called both when dequeueing and task _and_ during cgroup migrations.
> 
> Fix this by setting the flag in the uclamp_rq_inc_id() path to ensure
> things remain symmetrical.
> 
> Reported-by: Rick Yiu <rickyiu@...gle.com>
> Signed-off-by: Quentin Perret <qperret@...gle.com>

Argh, and I think this wants

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