Re: [PATCH v5 09/14] sched: Add over-utilization/tipping point indicator

[Vincent Guittot]

On Mon, 6 Aug 2018 at 12:08, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
>
> On 08/06/2018 10:40 AM, Vincent Guittot wrote:
> > On Fri, 3 Aug 2018 at 17:55, Quentin Perret <quentin.perret@xxxxxxx> wrote:
> >>
> >> On Friday 03 Aug 2018 at 15:49:24 (+0200), Vincent Guittot wrote:
> >>> On Fri, 3 Aug 2018 at 10:18, Quentin Perret <quentin.perret@xxxxxxx> wrote:
> >>>>
> >>>> On Friday 03 Aug 2018 at 09:48:47 (+0200), Vincent Guittot wrote:
> >>>>> On Thu, 2 Aug 2018 at 18:59, Quentin Perret <quentin.perret@xxxxxxx> wrote:
>
> [...]
>
> >> I think we're discussing two different things right now:
> >>      1. Should forkees go in find_energy_efficient_cpu() ?
> >>      2. Should forkees have 0 of initial util_avg when EAS is enabled ?
> >
> > It's the same topic: How EAS should consider a newly created task ?
> >
> > For now, we let the "performance" mode selects a CPU. This CPU will
> > most probably be worst CPU from a EAS pov because it's the idlest CPU
> > in the idlest group which is the opposite of what EAS tries to do
> >
> > The current behavior is :
> > For every new task, the cpu selection is done assuming it's a heavy
> > task with the max possible load_avg,  and it looks for the idlest cpu.
> > This means that if the system is lightly loaded, scheduler will select
> > most probably a idle big core.
>
> AFAICS, task load doesn't seem to be used for find_idlest_cpu() (
> find_idlest_group() and find_idlest_group_cpu()). So the forkee
> (SD_BALANCE_FORK) is placed independently of his task load.

hmm ... so what is used if load or runnable load are not used ?
find_idlest_group() uses load and runnable load but skip spare
capacity in case of fork

> Task load (task_h_load(p)) is used in
> wake_affine()->wake_affine_weight() but for this to be called it has to
> be a wakeup (SD_BALANCE_WAKE).
>
> > The utilization of this new task is then set to half of the remaining
> > capacity of the selected CPU which means that the idlest you are, the
> > biggest the task will be initialized to. This can easily be half a big
> > core which can be bigger than the max capacity of a little like on
> > hikey960. Then, util_est  will keep track of this value for a while
> > which will make this task like a big one.
>
> [...]
>