linux-kernel - Re: [PATCH v5 10/12] sched: get CPU's utilization statistic

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Date:	Mon, 15 Sep 2014 20:28:50 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"efault@....de" <efault@....de>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>
Subject: Re: [PATCH v5 10/12] sched: get CPU's utilization statistic

On Thu, Sep 11, 2014 at 01:34:12PM +0100, Peter Zijlstra wrote:
> > @@ -4514,6 +4519,17 @@ static int select_idle_sibling(struct task_struct *p, int target)
> >  	return target;
> >  }
> >  
> > +static int get_cpu_utilization(int cpu)
> > +{
> > +	unsigned long usage = cpu_rq(cpu)->cfs.usage_load_avg;
> > +	unsigned long capacity = capacity_of(cpu);
> > +
> > +	if (usage >= SCHED_LOAD_SCALE)
> > +		return capacity + 1;
> > +
> > +	return (usage * capacity) >> SCHED_LOAD_SHIFT;
> > +}
> 
> So if I understood patch 9 correct, your changelog is iffy.
> usage_load_avg should never get > 1 (of whatever unit), no matter how
> many tasks are on the rq. You can only maximally run all the time.
> 
> Therefore I can only interpret the if (usage >= SCHED_LOAD_SCALE) as
> numerical error handling, nothing more.

That is not entirely true unless you also classify transient usage
spikes due to task migrations as numerical errors as well.

Since each task sched_entity is carrying around 350ms worth of execution
history with it between different cpus and cpu utilization is based on
the sum of task entity usage_avg_contrib on the runqueue you may get
cfs.usage_load_avg > 1 temporarily after task migrations. It will
eventually converge to 1.

The same goes for new tasks which are initialized to have a
usage_avg_contrib of 1 and may be queued on cpu with tasks already
running. In that case cfs.usage_load_avg is temporarily unbounded.

> Also I'm not entirely sure I like the usage, utilization names/metrics.
> I would suggest to reverse them. Call the pure running number
> 'utilization' and this scaled with capacity 'usage' or so.

I can agree with calling running for utilization, but I'm not convienced
about capacity. What does it exactly cover here? I'm confused and
jetlagged.

Morten

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