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

[Hillf Danton <hdanton@sina.com>]

On Tue, 2 Dec 2025 14:01:39 +0100 Vincent Guittot wrote:
>On Tue, 2 Dec 2025 at 10:45, Hillf Danton <hdanton@sina.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@linaro.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
>
Given task queued on rq, I find the correct phrase, stack, in the cover
letter instead of stuck, and the long-standing stacking tasks mean load
balancer fails to cure that stack. 1/7 fixes that failure, no?