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

[Quentin Perret]

On Thursday 02 Aug 2018 at 18:07:49 (+0200), Vincent Guittot wrote:
> On Thu, 2 Aug 2018 at 18:00, Quentin Perret <quentin.perret@xxxxxxx> wrote:
> >
> > On Thursday 02 Aug 2018 at 17:55:24 (+0200), Vincent Guittot wrote:
> > > On Thu, 2 Aug 2018 at 17:30, Quentin Perret <quentin.perret@xxxxxxx> wrote:
> > > >
> > > > On Thursday 02 Aug 2018 at 17:14:15 (+0200), Vincent Guittot wrote:
> > > > > On Thu, 2 Aug 2018 at 16:14, Quentin Perret <quentin.perret@xxxxxxx> wrote:
> > > > > > Good point, setting the util_avg to 0 for new tasks should help
> > > > > > filtering out those tiny tasks too. And that would match with the idea
> > > > > > of letting tasks build their history before looking at their util_avg ...
> > > > > >
> > > > > > But there is one difference w.r.t frequency selection. The current code
> > > > > > won't mark the system overutilized, but will let sugov raise the
> > > > > > frequency when a new task is enqueued. So in case of a fork bomb, we
> > > > >
> > > > > If the initial value of util_avg is 0, we should not have any impact
> > > > > on the util_avg of the cfs rq on which the task is attached, isn't it
> > > > > ? so this should not impact both the over utilization state and the
> > > > > frequency selected by sugov or I'm missing something ?
> > > >
> > > > What I tried to say is that setting util_avg to 0 for new tasks will
> > > > prevent schedutil from raising the frequency in case of a fork bomb, and
> > > > I think that could be an issue. And I think this isn't an issue with the
> > > > patch as-is ...
> > >
> > > ok. So you also want to deal with fork bomb
> > > Not sure that you don't have some problem with current proposal too
> > > because select_task_rq_fair will always return prev_cpu because
> > > util_avg and util_est are 0 at that time
> >
> > But find_idlest_cpu() should select a CPU using load in case of a forkee
> > no ?
> 
> So you have to wait for the next tick that will set the overutilized
> and disable the want_energy. Until this point, all new tasks will be
> put on the current cpu

want_energy should always be false for forkees, because we set it only
for SD_BALANCE_WAKE.