Re: [RFC PATCH v4 08/12] sched: Add over-utilization/tipping point indicator

From: Quentin Perret <quentin.perret@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: rjw@rjwysocki.net, linux-kernel@vger.kernel.org,
	linux-pm@vger.kernel.org, gregkh@linuxfoundation.org,
	mingo@redhat.com, dietmar.eggemann@arm.com,
	morten.rasmussen@arm.com, chris.redpath@arm.com,
	patrick.bellasi@arm.com, valentin.schneider@arm.com,
	vincent.guittot@linaro.org, thara.gopinath@linaro.org,
	viresh.kumar@linaro.org, tkjos@google.com,
	joel@joelfernandes.org, smuckle@google.com, adharmap@quicinc.com,
	skannan@quicinc.com, pkondeti@codeaurora.org,
	juri.lelli@redhat.com, edubezval@gmail.com,
	srinivas.pandruvada@linux.intel.com, currojerez@riseup.net,
	javi.merino@kernel.org
Subject: Re: [RFC PATCH v4 08/12] sched: Add over-utilization/tipping point indicator
Date: Fri, 6 Jul 2018 14:20:57 +0100	[thread overview]
Message-ID: <20180706132056.GA17577@e108498-lin.cambridge.arm.com> (raw)
In-Reply-To: <20180706113208.GS2458@hirez.programming.kicks-ass.net>

On Friday 06 Jul 2018 at 13:32:08 (+0200), Peter Zijlstra wrote:
> On Thu, Jun 28, 2018 at 12:40:39PM +0100, Quentin Perret wrote:
> > @@ -5384,6 +5402,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> >  {
> >  	struct cfs_rq *cfs_rq;
> >  	struct sched_entity *se = &p->se;
> > +	int task_new = !(flags & ENQUEUE_WAKEUP);
> >  
> >  	/*
> >  	 * The code below (indirectly) updates schedutil which looks at
> > @@ -5431,8 +5450,12 @@ 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);
> > +		if (!task_new)
> > +			update_overutilized_status(rq);
> 
> I'm confused... why only for !ENQUEUE_WAKEUP

So, what we try to do here is to _not_ set the overutilized flag based
on the utilization of a new task, because its utilization is 'wrong'.
That should help avoiding spurious transitions above the overutilized
threshold.

Was there also something else Morten ?

> and why use a local
> variable for something that's used only once?

No good reasons. An older version of this patch used 'task_new' in two
different locations. This is no longer true, I'll clean that up for v5.

Thanks,
Quentin