linux-kernel - Re: [PATCH v2] sched/uclamp: Align uclamp and util

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Message-ID: <CAKfTPtD2T8Qm-6Nq-9qODQASoU9Wn1Ujk4uQbbN1Dvo3+EFZFA@mail.gmail.com>
Date: Wed, 16 Apr 2025 11:42:40 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Xuewen Yan <xuewen.yan94@...il.com>
Cc: Xuewen Yan <xuewen.yan@...soc.com>, dietmar.eggemann@....com, mingo@...hat.com, 
	peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	hongyan.xia2@....com, qyousef@...alina.io, ke.wang@...soc.com, 
	di.shen@...soc.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/uclamp: Align uclamp and util_est and call
 before freq update

On Wed, 16 Apr 2025 at 04:55, Xuewen Yan <xuewen.yan94@...il.com> wrote:
>
> On Wed, Apr 16, 2025 at 1:05 AM Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
> >
> > On Tue, 25 Mar 2025 at 02:48, Xuewen Yan <xuewen.yan@...soc.com> wrote:
> > >
> > > When task's uclamp is set, we hope that the CPU frequency
> > > can increase as quickly as possible when the task is enqueued.
> > > Because the cpu frequency updating happens during the enqueue_task(),
> >
> > Strictly speaking, it doesn't happen during enqueue_task but when :
> > - attach/detach tasks when migrating
> > - update_load_avg decayed
> > - io_wait
> >
> > This often happens during an enqueue but not always ...
>
> Okay, I would make some adjustments to these descriptions.
>
> >
> > > so the rq's uclamp needs to be updated before the task is enqueued,
> >
> > this doesn't ensure that new rq's uclamp will be taken into account
>
> Did I miss something?
>
> As following stack:
> enqueue_task_fair()
> update_load_avg()
> cfs_rq_util_change(cfs_rq, 0);

As mentioned above, this doesn't always happen so you are not ensured
to take uclamp into account. If you mandate to take uclamp value into
account immediately this is not enough

> cpufreq_update_util()
>   sugov_update_shared()
>     sugov_next_freq_shared()
>      sugov_get_util()
>         effective_cpu_util()
>             *min = max(irq + cpu_bw_dl(rq), uclamp_rq_get(rq, UCLAMP_MIN));
>             *max = min(scale, uclamp_rq_get(rq, UCLAMP_MAX));
>
> So, the rq's uclamp value should update before enqueue_task().
> >
> > > just like util_est.
> >
> > just like util_est
> >
> > > So, aline the uclamp and util_est and call before freq update.
> >
> > nit s/aline/align/ ?
> align.
> >
> > >
> > > For sched-delayed tasks, the rq uclamp/util_est should only be updated
> > > when they are enqueued upon being awakened.
> > > So simply the logic of util_est's enqueue/dequeue check.
> > >
> > > Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> > > ---
> > > v2:
> > > - simply the util-est's en/dequeue check;
> > > ---
> > > Previous discussion:
> > > https://lore.kernel.org/all/CAB8ipk8pEvOtCm-d0o1rsekwxPWUHk9iBGtt9TLTWW-iWTQKiA@mail.gmail.com/
> > > https://lore.kernel.org/all/84441660bef0a5e67fd09dc3787178d0276dad31.1740664400.git.hongyan.xia2@arm.com/T/#u
> > > https://lore.kernel.org/all/CAB8ipk9LpbiUDnbcV6+59+Sa=Ai7tFzO===mpLD3obNdV4=J-A@mail.gmail.com/T/#u
> > > https://lore.kernel.org/all/aa8baf67-a8ec-4ad8-a6a8-afdcd7036771@arm.com/
> > > ---
> > >  kernel/sched/core.c | 17 ++++++++++-------
> > >  kernel/sched/fair.c |  4 ++--
> > >  2 files changed, 12 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 042351c7afce..72fbe2031e54 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -1747,7 +1747,7 @@ static inline void uclamp_rq_dec_id(struct rq *rq, struct task_struct *p,
> > >         }
> > >  }
> > >
> > > -static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
> > > +static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p, int flags)
> > >  {
> > >         enum uclamp_id clamp_id;
> > >
> > > @@ -1763,7 +1763,8 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
> > >         if (unlikely(!p->sched_class->uclamp_enabled))
> > >                 return;
> > >
> > > -       if (p->se.sched_delayed)
> > > +       /* Only inc the delayed task which being woken up. */
> > > +       if (p->se.sched_delayed && !(flags & ENQUEUE_DELAYED))
> > >                 return;
> > >
> > >         for_each_clamp_id(clamp_id)
> > > @@ -2031,7 +2032,7 @@ static void __init init_uclamp(void)
> > >  }
> > >
> > >  #else /* !CONFIG_UCLAMP_TASK */
> > > -static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p) { }
> > > +static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p, int flags) { }
> > >  static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p) { }
> > >  static inline void uclamp_fork(struct task_struct *p) { }
> > >  static inline void uclamp_post_fork(struct task_struct *p) { }
> > > @@ -2067,12 +2068,14 @@ void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
> > >         if (!(flags & ENQUEUE_NOCLOCK))
> > >                 update_rq_clock(rq);
> > >
> > > -       p->sched_class->enqueue_task(rq, p, flags);
> > >         /*
> > > -        * Must be after ->enqueue_task() because ENQUEUE_DELAYED can clear
> > > -        * ->sched_delayed.
> > > +        * Can be before ->enqueue_task() because uclamp considers the
> > > +        * ENQUEUE_DELAYED task before its ->sched_delayed gets cleared
> > > +        * in ->enqueue_task().
> > >          */
> > > -       uclamp_rq_inc(rq, p);
> > > +       uclamp_rq_inc(rq, p, flags);
> > > +
> > > +       p->sched_class->enqueue_task(rq, p, flags);
> > >
> > >         psi_enqueue(p, flags);
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index c798d2795243..c92fee07fb7b 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -6930,7 +6930,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> > >          * Let's add the task's estimated utilization to the cfs_rq's
> > >          * estimated utilization, before we update schedutil.
> > >          */
> > > -       if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags & ENQUEUE_RESTORE))))
> > > +       if (!p->se.sched_delayed || (flags & ENQUEUE_DELAYED))
> >
> > commit message doesn't explain why you change util_est condition
>
> Because, the sched_delayed flag is set when dequeue_entity, and clear
> after the condition,
> so for migrating/prio_change, we could just check the sched_delayed.

Why is testing sched_delayed enough for migrating/prio_change ?
With your change, we will remove then add back util_est when changing
prio of the task which is useless


> And for the wakeup, because the the sched_delayed flag is cleared after this,
> so use the ENQUEUE_DELAYED flag to ensure the util_est could enqueue.
>
> >
> > >                 util_est_enqueue(&rq->cfs, p);
> > >
> > >         if (flags & ENQUEUE_DELAYED) {
> > > @@ -7168,7 +7168,7 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
> > >   */
> > >  static bool dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> > >  {
> > > -       if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags & DEQUEUE_SAVE))))
> > > +       if (!p->se.sched_delayed)
> >
> > same here, you should explain in commit message why it's okay to do so
>
> Same as above,  the sched_delayed flag is set when dequeue_entity, so
> this place,
> the sched_delayed was not set when sleeping, If the flag is set, it
> indicates that it
> was migrating or prio changing.
>
> By the way, I will kindly update these reasons in the commit message.
>
> Thanks!
>
> BR
> ---
> xuewen