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

[Quentin Perret <quentin.perret@arm.com>]

On Thursday 02 Aug 2018 at 14:26:29 (+0200), Peter Zijlstra wrote:
> On Tue, Jul 24, 2018 at 01:25:16PM +0100, Quentin Perret wrote:
> > @@ -5100,8 +5118,17 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> >  		update_cfs_group(se);
> >  	}
> >  
> > -	if (!se)
> > +	if (!se) {
> >  		add_nr_running(rq, 1);
> > +		/*
> > +		 * The utilization of a new task is 'wrong' so wait for it
> > +		 * to build some utilization history before trying to detect
> > +		 * the overutilized flag.
> > +		 */
> > +		if (flags & ENQUEUE_WAKEUP)
> > +			update_overutilized_status(rq);
> > +
> > +	}
> >  
> >  	hrtick_update(rq);
> >  }
> 
> That is a somewhat dodgy hack. There is no guarantee what so ever that
> when the task wakes next its history is any better. The comment doesn't
> reflect this I feel.

AFAICT the main use-case here is to avoid re-enabling the load balance
and ruining all the task placement because of a tiny task. I don't
really see how we can do that differently ...

Or am I missing something Morten ?

In the meantime, I can try to improve the comment at least :-)

Thanks,
Quentin