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Message-ID: <CAKfTPtCm=hoWNG5pUW+tMWgg=Dg1c-=BTTkW=jRirC1P2MycvA@mail.gmail.com>
Date: Mon, 1 Dec 2025 20:33:01 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Christian Loehle <christian.loehle@....com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org,
pierre.gondois@....com, kprateek.nayak@....com, qyousef@...alina.io,
hongyan.xia2@....com, luis.machado@....com
Subject: Re: [RFC PATCH 6/6 v7] sched/fair: Add EAS and idle cpu push trigger
On Mon, 1 Dec 2025 at 18:49, Vincent Guittot <vincent.guittot@...aro.org> wrote:
>
> On Mon, 1 Dec 2025 at 14:53, Christian Loehle <christian.loehle@....com> wrote:
> >
> > Some nits below for now
> >
> > On 12/1/25 09:13, 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
> >
> > worth
> > the
>
> +1
>
> >
> > > 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);
> > > +
> >
> > This doesn't belong here
>
> yes, don't know what I mess up with my patches
>
> >
> > > /*
> > > * 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;
> > > +
> > > + 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 (sched_idle_push_task(p, rq))
> > > + return true;
> > > +
> > > return false;
> > > }
> > >
> > > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > > index cf643a5ddedd..5edf7b117ed9 100644
> > > --- a/kernel/sched/topology.c
> > > +++ b/kernel/sched/topology.c
> > > @@ -391,10 +391,13 @@ static void sched_energy_set(bool has_eas)
> > > if (sched_debug())
> > > pr_info("%s: stopping EAS\n", __func__);
> > > static_branch_disable_cpuslocked(&sched_energy_present);
> > > + static_branch_dec_cpuslocked(&sched_push_task);
> > > + } else if (has_eas && !sched_energy_enabled()) {
> > > } else if (has_eas && !static_branch_unlikely(&sched_energy_present)) {
> >
> > This could just be (has_eas && && sched_energy_enabled() && !static_branch_unlikely(&sched_energy_present))
> > to avoid the awkward else if above
>
> Argh, I messed up something with this patchset and another pending
> cleanup patch when I rebased it.
> It should be :
>
> static_branch_disable_cpuslocked(&sched_energy_present);
> + static_branch_dec_cpuslocked(&sched_push_task);
> } else if (has_eas && !static_branch_unlikely(&sched_energy_present)) {
>
> I need to rerun the bench to check that the results of the cover
> letter are still correct.
And the results are now the same
Sorry for the noise, I'm going to fix this in a v8
>
> That's what happens when you want to send a patchset too quickly ...
>
>
> >
> > > if (sched_debug())
> > > pr_info("%s: starting EAS\n", __func__);
> > > static_branch_enable_cpuslocked(&sched_energy_present);
> > > + static_branch_inc_cpuslocked(&sched_push_task);
> > > }
> > > }
> > >
> >
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