linux-kernel - Re: [PATCH v3 1/3] sched: Fix UCLAMP_FLAG

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Message-ID: <YN7+X+5NviroSqZY@google.com>
Date:   Fri, 2 Jul 2021 11:54:07 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....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 v3 1/3] sched: Fix UCLAMP_FLAG_IDLE setting

On Thursday 01 Jul 2021 at 18:59:32 (+0100), Qais Yousef wrote:
> On 07/01/21 15:20, Quentin Perret wrote:
> > > > Right or maybe we can just check that uclamp_id == UCLAMP_MAX here and
> > > > we should be good to go? That is, what about the below?
> > > 
> > > Wouldn't it be better to do this from uclamp_idle_reset() then?
> > 
> > That should work too, but clearing the flag outside of
> > uclamp_rq_inc_id() feels a little bit more robust to ordering
> > issues.
> > 
> > Specifically, uclamp_rq_inc() has the following pattern:
> > 
> > 	for_each_clamp_id(clamp_id)
> > 		uclamp_rq_inc_id(rq, p , clamp_id);
> > 
> > 	if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
> > 		rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
> > 
> > So, if we change this to clear the flag from
> > uclamp_rq_inc_id()->uclamp_idle_reset() then we'll have issues if
> > (for example) for_each_clamp_id()'s order changes in the future.
> > IOW, it feels cleaner to not create side effects in uclamp_rq_inc_id()
> > that impact the idle flag given that its very own behaviour depends on
> > the flag.
> > 
> > WDYT?
> 
> Do the clearing from outside the loop then to keep the pattern consistent?

Right, but I actually preferred doing it from here as we're under
task_rq_lock(), which means well behaved readers won't observe the flag
being transiently set. I could also refactor the locking, but oh well ...

> Anyway, I think there's no clear objective advantage. So I'll trust your
> judgement and promise not to complain with your final choice ;-)

:) Alrighty, I'll cook something.

Thanks!
Quentin