Re: [PATCH v2 1/2] sched/fair: Take thermal pressure into account while estimating energy

[Vincent Guittot]

On Thu, 10 Jun 2021 at 11:36, Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
>
>
>
> On 6/10/21 10:11 AM, Vincent Guittot wrote:
> > On Thu, 10 Jun 2021 at 10:42, Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
> >>
> >>
> >>
> >> On 6/10/21 8:59 AM, Vincent Guittot wrote:
> >>> On Fri, 4 Jun 2021 at 10:10, Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
> >>>>
> >>>> Energy Aware Scheduling (EAS) needs to be able to predict the frequency
> >>>> requests made by the SchedUtil governor to properly estimate energy used
> >>>> in the future. It has to take into account CPUs utilization and forecast
> >>>> Performance Domain (PD) frequency. There is a corner case when the max
> >>>> allowed frequency might be reduced due to thermal. SchedUtil is aware of
> >>>> that reduced frequency, so it should be taken into account also in EAS
> >>>> estimations.
> >>>>
> >>>> SchedUtil, as a CPUFreq governor, knows the maximum allowed frequency of
> >>>> a CPU, thanks to cpufreq_driver_resolve_freq() and internal clamping
> >>>> to 'policy::max'. SchedUtil is responsible to respect that upper limit
> >>>> while setting the frequency through CPUFreq drivers. This effective
> >>>> frequency is stored internally in 'sugov_policy::next_freq' and EAS has
> >>>> to predict that value.
> >>>>
> >>>> In the existing code the raw value of arch_scale_cpu_capacity() is used
> >>>> for clamping the returned CPU utilization from effective_cpu_util().
> >>>> This patch fixes issue with too big single CPU utilization, by introducing
> >>>> clamping to the allowed CPU capacity. The allowed CPU capacity is a CPU
> >>>> capacity reduced by thermal pressure signal. We rely on this load avg
> >>>> geometric series in similar way as other mechanisms in the scheduler.
> >>>>
> >>>> Thanks to knowledge about allowed CPU capacity, we don't get too big value
> >>>> for a single CPU utilization, which is then added to the util sum. The
> >>>> util sum is used as a source of information for estimating whole PD energy.
> >>>> To avoid wrong energy estimation in EAS (due to capped frequency), make
> >>>> sure that the calculation of util sum is aware of allowed CPU capacity.
> >>>>
> >>>> Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>
> >>>> ---
> >>>>    kernel/sched/fair.c | 17 ++++++++++++++---
> >>>>    1 file changed, 14 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >>>> index 161b92aa1c79..1aeddecabc20 100644
> >>>> --- a/kernel/sched/fair.c
> >>>> +++ b/kernel/sched/fair.c
> >>>> @@ -6527,6 +6527,7 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
> >>>>           struct cpumask *pd_mask = perf_domain_span(pd);
> >>>>           unsigned long cpu_cap = arch_scale_cpu_capacity(cpumask_first(pd_mask));
> >>>>           unsigned long max_util = 0, sum_util = 0;
> >>>> +       unsigned long _cpu_cap = cpu_cap;
> >>>>           int cpu;
> >>>>
> >>>>           /*
> >>>> @@ -6558,14 +6559,24 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
> >>>>                                   cpu_util_next(cpu, p, -1) + task_util_est(p);
> >>>>                   }
> >>>>
> >>>> +               /*
> >>>> +                * Take the thermal pressure from non-idle CPUs. They have
> >>>> +                * most up-to-date information. For idle CPUs thermal pressure
> >>>> +                * signal is not updated so often.
> >>>
> >>> What do you mean by "not updated so often" ? Do you have a value ?
> >>>
> >>> Thermal pressure is updated at the same rate as other PELT values of
> >>> an idle CPU. Why is it a problem there ?
> >>>
> >>
> >>
> >> For idle CPU the value is updated 'remotely' by some other CPU
> >> running nohz_idle_balance(). That goes into
> >> update_blocked_averages() if the flags and checks are OK inside
> >> update_nohz_stats(). Sometimes this is not called
> >> because other_have_blocked() returned false. It can happen for a long
> >
> > So i miss that you were in a loop and the below was called for each
> > cpu and _cpu_cap was overwritten
> >
> > +               if (!idle_cpu(cpu))
> > +                       _cpu_cap = cpu_cap - thermal_load_avg(cpu_rq(cpu));
> >
> > But that also means that if the 1st cpus of the pd are idle, they will
> > use original capacity whereas the other ones will remove the thermal
> > pressure. Isn't this a problem  ?  You don't use the same capacity for
> > all cpus in the performance domain regarding the thermal pressure?
>
> True, but in the experiments for idle CPUs I haven't
> observed that they still have some big util (bigger than _cpu_cap).
> It decayed already, so it's not a problem for idle CPUs.
>
> Although, it might be my test case which didn't trigger something.
> Is it worth to add the loop above this one, to be 100% sure and
> get a thermal pressure signal from some running CPU?
> Then apply the same value always inside the 2nd loop?

Either it's a problem and you must make sure to use the same capacity
for all cpus of a PD

Or it's not but in this case you don't need  if (!idle_cpu(cpu))