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Date:   Fri, 11 Jun 2021 07:25:42 +0000
From:   Quentin Perret <qperret@google.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     mingo@redhat.com, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, qais.yousef@arm.com, rickyiu@google.com,
        wvw@google.com, patrick.bellasi@matbug.net, xuewen.yan94@gmail.com,
        linux-kernel@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH v2 1/3] sched: Fix UCLAMP_FLAG_IDLE setting
Message-ID: <YMMP9uqcCeDlt95F@google.com>
References: <20210610151306.1789549-1-qperret@google.com>
 <20210610151306.1789549-2-qperret@google.com>
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On Thursday 10 Jun 2021 at 21:05:12 (+0200), Peter Zijlstra wrote:
> On Thu, Jun 10, 2021 at 03:13:04PM +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 tasks on the rq.
> > Specifically, the flag is cleared in the uclamp_rq_inc() path, which is
> > called at enqueue time, but set in uclamp_rq_dec_id() which is called
> > both when dequeueing a task _and_ in the update_uclamp_active() path. As
> > a result, when both uclamp_rq_{dec,ind}_id() are called from
> > update_uclamp_active(), the flag ends up being set but not cleared,
> > hence leaving the runqueue in a broken state.
> > 
> > Fix this by setting the flag in the uclamp_rq_inc_id() path to ensure
> > things remain symmetrical.
> 
> The code you moved is neither in uclamp_rq_inc_id(), although
> uclamp_idle_reset() is called from there

Yep, that is what I was trying to say.

> nor does it _set_ the flag.

Ahem. That I don't have a good excuse for ...
> 
> I'm thinking it's been a long warm day? ;-)

Indeed :-)

Let me have another cup of coffee and try to write this again.

Thanks,
Quentin