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

[Quentin Perret]

On Thursday 02 Aug 2018 at 15:08:01 (+0200), Peter Zijlstra wrote:
> On Thu, Aug 02, 2018 at 02:03:38PM +0100, Quentin Perret wrote:
> > 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 ...
> 
> Sure I realize that.. but it doesn't completely avoid it. Suppose this
> new task instantly blocks and wakes up again. Then its util signal will
> be exactly what you didn't want but we'll account it and cause the above
> scenario you wanted to avoid.

That is true. ... I also realize now that this patch was written long
before util_est, and that also has an impact here, especially in the
scenario you described where the task blocks. So any wake-up after the
first enqueue will risk to overutilize the system, even if the task
blocked for ages.

Hmm ...