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

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Message-ID: <20210701110803.2lka3eaoukbb6b4p@e107158-lin.cambridge.arm.com>
Date:   Thu, 1 Jul 2021 12:08:03 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Quentin Perret <qperret@...gle.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 07/01/21 10:07, Quentin Perret wrote:
> On Wednesday 30 Jun 2021 at 15:45:14 (+0000), Quentin Perret wrote:
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index b094da4c5fea..c0b999a8062a 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -980,7 +980,6 @@ static inline void uclamp_idle_reset(struct rq *rq, enum uclamp_id clamp_id,
> >         if (!(rq->uclamp_flags & UCLAMP_FLAG_IDLE))
> >                 return;
> > 
> > -       rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
> >         WRITE_ONCE(rq->uclamp[clamp_id].value, clamp_value);
> >  }
> > 
> > @@ -1253,6 +1252,10 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
> > 
> >         for_each_clamp_id(clamp_id)
> >                 uclamp_rq_inc_id(rq, p, clamp_id);
> > +
> > +       /* Reset clamp idle holding when there is one RUNNABLE task */
> > +       if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
> > +               rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
> >  }
> > 
> >  static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p)
> > @@ -1300,6 +1303,13 @@ uclamp_update_active(struct task_struct *p, enum uclamp_id clamp_id)
> >         if (p->uclamp[clamp_id].active) {
> >                 uclamp_rq_dec_id(rq, p, clamp_id);
> >                 uclamp_rq_inc_id(rq, p, clamp_id);
> > +
> > +               /*
> > +                * Make sure to clear the idle flag if we've transiently reached
> > +                * 0 uclamp active tasks on the rq.
> > +                */
> > +               if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
> > +                       rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
> 
> Bah, now that I had coffee I realize this has the exact same problem.
> Let me look at this again ...

Hehe uclamp has this effect. It's all obvious, until it's not :-)

Yes this needs to be out of the loop.

Thanks for looking at this!

Cheers

--
Qais Yousef