linux-kernel - Re: [PATCH v6 4/6] sched: get CPU's usage statistic

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Message-ID: <20140926155803.GA23693@e103034-lin>
Date:	Fri, 26 Sep 2014 16:58:03 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	"peterz@...radead.org" <peterz@...radead.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>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"bsegall@...gle.com" <bsegall@...gle.com>
Subject: Re: [PATCH v6 4/6] sched: get CPU's usage statistic

On Fri, Sep 26, 2014 at 01:17:43PM +0100, Vincent Guittot wrote:
> On 25 September 2014 21:05, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> > On 23/09/14 17:08, Vincent Guittot wrote:
> >> Monitor the usage level of each group of each sched_domain level. The usage is
> >> the amount of cpu_capacity that is currently used on a CPU or group of CPUs.
> >> We use the utilization_load_avg to evaluate the usage level of each group.
> >>
> >> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> >> ---
> >>  kernel/sched/fair.c | 13 +++++++++++++
> >>  1 file changed, 13 insertions(+)
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index 2cf153d..4097e3f 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -4523,6 +4523,17 @@ static int select_idle_sibling(struct task_struct *p, int target)
> >>       return target;
> >>  }
> >>
> >> +static int get_cpu_usage(int cpu)
> >> +{
> >> +     unsigned long usage = cpu_rq(cpu)->cfs.utilization_load_avg;
> >> +     unsigned long capacity = capacity_orig_of(cpu);
> >> +
> >> +     if (usage >= SCHED_LOAD_SCALE)
> >> +             return capacity + 1;
> >
> > Why you are returning rq->cpu_capacity_orig + 1 (1025) in case
> > utilization_load_avg is greater or equal than 1024 and not usage or
> > (usage * capacity) >> SCHED_LOAD_SHIFT too?
> 
> The usage can't be higher than the full capacity of the CPU because
> it's about the running time on this CPU. Nevertheless, usage can be
> higher than SCHED_LOAD_SCALE because of unfortunate rounding in
> avg_period and running_load_avg or just after migrating tasks until
> the average stabilizes with the new running time.

I fully agree that the cpu usage should be capped to capacity, but why
do you return capacity + 1? I would just return capacity, no?

Now that you have gotten rid of 'usage' everywhere else, shouldn't this
function be renamed to get_cpu_utilization()?
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