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

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Date:   Thu, 1 Jul 2021 15:20:50 +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 15:57:50 (+0100), Qais Yousef wrote:
> On 07/01/21 12:43, Quentin Perret wrote:
> > On Thursday 01 Jul 2021 at 12:08:03 (+0100), Qais Yousef wrote:
> > > 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 :-)
> > 
> > Indeed ... :)
> > 
> > > Yes this needs to be out of the loop.
> > 
> > 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?

Cheers,
Quentin