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

[Vincent Guittot]

On Thu, 4 Dec 2025 at 07:59, Hillf Danton <hdanton@xxxxxxxx> wrote:
>
> On Wed, 3 Dec 2025 14:32:06 +0100 Vincent Guittot wrote:
> > On Wed, 3 Dec 2025 at 10:00, Hillf Danton <hdanton@xxxxxxxx> wrote:
> > > 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?
> >
> > It's not just stacked because we sometimes/often want to stack tasks
> > on the same CPU. EAS is based on the assumption that tasks will sleep
> > and wake up regularly and EAS will select a new CPU at each wakeup but
> > it's not always true. We can have situations where task A has been put
> > on CPU0when waking up, sharing the CPU with others tasks. But after
> > some time, task A should be better on CPUB now not because of not
> > fitting anymore on CPU0 but just because the system state has changed
> > since its wakeup. Because task A shares the CPU0 with other tasks, it
> > can takes dozen/hundreds of ms to finish its works and to sleep and we
> > don't wait those hundreds of ms whereas a CPU1 might be a better
> > choice now.
> >
> Even if task is pushed from an ARM little core to a big one, the net
> result could be zero, either because the number of stacking tasks on the
> dst CPU increases or more important the dst CPU cycles are shared at the
> pace of tick. In general if stacking is not mitigated but migrated from
> one CPU to another, pushing could not make much difference.

if select_task_rq/feec returns a new CPU, it means that it will make a
difference in the consumed energy or the available capacity for the
task. And when overutilized, it looks for an idle CPUs