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

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Date:   Thu, 1 Jul 2021 12:43:24 +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 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?


diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b094da4c5fea..8e9b8106a0df 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 active tasks on rq.
+                */
+               if (clamp_id == MAX && rq->uclamp_flags & UCLAMP_FLAG_IDLE)
+                       rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
        }

        task_rq_unlock(rq, p, &rf);