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

Morten Rasmussen
2014-09-26T15:57:42Z

On Fri, Sep 26, 2014 at 01:17:43PM +0100, Vincent Guittot wrote:
> On 25 September 2014 21:05, Dietmar Eggemann <dietmar.eggemann@arm.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@linaro.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()?