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

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Date:   Thu, 2 Aug 2018 14:18:51 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, gregkh@...uxfoundation.org,
        mingo@...hat.com, dietmar.eggemann@....com,
        morten.rasmussen@....com, chris.redpath@....com,
        patrick.bellasi@....com, valentin.schneider@....com,
        vincent.guittot@...aro.org, thara.gopinath@...aro.org,
        viresh.kumar@...aro.org, tkjos@...gle.com, joel@...lfernandes.org,
        smuckle@...gle.com, adharmap@...cinc.com, skannan@...cinc.com,
        pkondeti@...eaurora.org, juri.lelli@...hat.com,
        edubezval@...il.com, srinivas.pandruvada@...ux.intel.com,
        currojerez@...eup.net, javi.merino@...nel.org
Subject: Re: [PATCH v5 09/14] sched: Add over-utilization/tipping point
 indicator

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 ...