linux-kernel - Re: [RFC PATCH 6/6 v7] sched/fair: Add EAS and idle cpu push trigger

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Message-ID: <CAKfTPtCi3xw53_S8W2CZKZVE60q8G-AVq82UUm_SMD+Pt0xDYA@mail.gmail.com>
Date: Tue, 2 Dec 2025 14:01:39 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Hillf Danton <hdanton@...a.com>
Cc: peterz@...radead.org, linux-kernel@...r.kernel.org, pierre.gondois@....com, 
	kprateek.nayak@....com, qyousef@...alina.io, christian.loehle@....com, 
	luis.machado@....com
Subject: Re: [RFC PATCH 6/6 v7] sched/fair: Add EAS and idle cpu push trigger

On Tue, 2 Dec 2025 at 10:45, Hillf Danton <hdanton@...a.com> wrote:
>
> On Mon,  1 Dec 2025 10:13:08 +0100 Vincent Guittot wrote:
> > EAS is based on wakeup events to efficiently place tasks on the system, but
> > there are cases where a task doesn't have wakeup events anymore or at a far
> > too low pace. For such cases, we check if it's worht pushing hte task on
> > another CPUs instead of putting it back in the enqueued list.
> >
> > Wake up events remain the main way to migrate tasks but we now detect
> > situation where a task is stuck on a CPU by checking that its utilization
> > is larger than the max available compute capacity (max cpu capacity or
> > uclamp max setting)
> >
> > When the system becomes overutilized and some CPUs are idle, we try to
> > push tasks instead of waiting periodic load balance.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >  kernel/sched/fair.c     | 65 +++++++++++++++++++++++++++++++++++++++++
> >  kernel/sched/topology.c |  3 ++
> >  2 files changed, 68 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 9af8d0a61856..e9e1d0c05805 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6990,6 +6990,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> >  }
> >
> >  static void fair_remove_pushable_task(struct rq *rq, struct task_struct *p);
> > +
> >  /*
> >   * Basically dequeue_task_fair(), except it can deal with dequeue_entity()
> >   * failing half-way through and resume the dequeue later.
> > @@ -8499,8 +8500,72 @@ static inline bool sched_push_task_enabled(void)
> >       return static_branch_unlikely(&sched_push_task);
> >  }
> >
> > +static inline bool task_stuck_on_cpu(struct task_struct *p, int cpu)
> > +{
> > +     unsigned long max_capa, util;
> > +
> > +     max_capa = min(get_actual_cpu_capacity(cpu),
> > +                    uclamp_eff_value(p, UCLAMP_MAX));
> > +     util = max(task_util_est(p), task_runnable(p));
> > +
> > +     /*
> > +      * Return true only if the task might not sleep/wakeup because of a low
> > +      * compute capacity. Tasks, which wake up regularly, will be handled by
> > +      * feec().
> > +      */
> > +     return (util > max_capa);
> > +}
> > +
> > +static inline bool sched_energy_push_task(struct task_struct *p, struct rq *rq)
> > +{
> > +     if (!sched_energy_enabled())
> > +             return false;
> > +
> > +     if (is_rd_overutilized(rq->rd))
> > +             return false;
> > +
> > +     if (task_stuck_on_cpu(p, cpu_of(rq)))
> > +             return true;
> > +
> > +     if (!task_fits_cpu(p, cpu_of(rq)))
> > +             return true;
> > +
> > +     return false;
> > +}
> > +
> > +static inline bool sched_idle_push_task(struct task_struct *p, struct rq *rq)
> > +{
> > +     if (rq->nr_running == 1)
> > +             return false;
> > +
> > +     if (!is_rd_overutilized(rq->rd))
> > +             return false;
> > +
> > +     /* If there are idle cpus in the llc then try to push the task on it */
> > +     if (test_idle_cores(cpu_of(rq)))
> > +             return true;
> > +
> > +     return false;
> > +}
> > +
> > +
> >  static bool fair_push_task(struct rq *rq, struct task_struct *p)
> >  {
> > +     if (!task_on_rq_queued(p))
> > +             return false;
>
> Task is queued on rq.
> > +
> > +     if (p->se.sched_delayed)
> > +             return false;
> > +
> > +     if (p->nr_cpus_allowed == 1)
> > +             return false;
> > +
> > +     if (sched_energy_push_task(p, rq))
> > +             return true;
>
> If task is stuck on CPU, it could not be on rq. Weird.

May be it comes from my description and I should use task_stuck_on_rq
By stuck, I mean that the task doesn't have any opportunity to migrate
on another cpu/rq and stay "forever"  (at least until next sleep) on
this cpu/rq because load balancing is disabled/bypassed w/ EAS
Here Stuck does not mean blocked/sleeping

> > +
> > +     if (sched_idle_push_task(p, rq))
> > +             return true;
> > +
> >       return false;
> >  }
> >
> More, in the tick path,
>
> task_tick_fair
>   check_pushable_task
>     fair_push_task
>       task_on_rq_queued // this check makes no sense

I want to use a single entry point (fair_push_task) for deciding to
push a task so I agree that testing task_on_rq_queued() at tick is
useless but it is needed for other cases when the task is put back in
the rb tree