linux-kernel - Re: [PATCH v2 1/3] sched: Fix UCLAMP_FLAG

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Date:   Fri, 11 Jun 2021 07:25:42 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, qais.yousef@....com, rickyiu@...gle.com,
        wvw@...gle.com, patrick.bellasi@...bug.net, xuewen.yan94@...il.com,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v2 1/3] sched: Fix UCLAMP_FLAG_IDLE setting

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