Re: [PATCH v5 05/10] cpufreq/schedutil: get max utilization

Vincent Guittot
2018-05-28T15:03:11Z

Hi Juri,

On 28 May 2018 at 12:12, Juri Lelli <juri.lelli@redhat.com> wrote:
> Hi Vincent,
>
> On 25/05/18 15:12, Vincent Guittot wrote:
>> Now that we have both the dl class bandwidth requirement and the dl class
>> utilization, we can use the max of the 2 values when agregating the
>> utilization of the CPU.
>>
>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
>> ---
>> kernel/sched/sched.h | 6 +++++-
>> 1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 4526ba6..0eb07a8 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -2194,7 +2194,11 @@ static inline void cpufreq_update_util(struct rq *rq, unsigned int flags) {}
>> #ifdef CONFIG_CPU_FREQ_GOV_SCHEDUTIL
>> static inline unsigned long cpu_util_dl(struct rq *rq)
>> {
>> - return (rq->dl.running_bw * SCHED_CAPACITY_SCALE) >> BW_SHIFT;
>> + unsigned long util = (rq->dl.running_bw * SCHED_CAPACITY_SCALE) >> BW_SHIFT;
>
> I'd be tempted to say the we actually want to cap to this one above
> instead of using the max (as you are proposing below) or the
> (theoretical) power reduction benefits of using DEADLINE for certain
> tasks might vanish.

The problem that I'm facing is that the sched_entity bandwidth is
removed after the 0-lag time and the rq->dl.running_bw goes back to
zero but if the DL task has preempted a CFS task, the utilization of
the CFS task will be lower than reality and schedutil will set a lower
OPP whereas the CPU is always running. The example with a RT task
described in the cover letter can be run with a DL task and will give
similar results.
avg_dl.util_avg tracks the utilization of the rq seen by the scheduler
whereas rq->dl.running_bw gives the minimum to match DL requirement.

>
>> +
>> + util = max_t(unsigned long, util, READ_ONCE(rq->avg_dl.util_avg));
>> +
>> + return util;
>
> Anyway, just a quick thought. I guess we should experiment with this a
> bit. Now, I don't unfortunately have a Arm platform at hand for testing.
> Claudio, Luca (now Cc-ed), would you be able to fire some tests with
> this change?
>
> Oh, adding Joel and Alessio as well that experimented with DEADLINE
> lately.
>
> Thanks,
>
> - Juri