Messages in this thread | | | Subject | Re: [PATCH v2 1/2] sched/fair: Take thermal pressure into account while estimating energy | From | Lukasz Luba <> | Date | Thu, 10 Jun 2021 10:52:21 +0100 |
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On 6/10/21 10:41 AM, Vincent Guittot wrote: > On Thu, 10 Jun 2021 at 11:36, Lukasz Luba <lukasz.luba@arm.com> wrote: >> >> >> >> On 6/10/21 10:11 AM, Vincent Guittot wrote: >>> On Thu, 10 Jun 2021 at 10:42, Lukasz Luba <lukasz.luba@arm.com> wrote: >>>> >>>> >>>> >>>> On 6/10/21 8:59 AM, Vincent Guittot wrote: >>>>> On Fri, 4 Jun 2021 at 10:10, Lukasz Luba <lukasz.luba@arm.com> 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@arm.com> >>>>>> --- >>>>>> 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. > > But it's a problem because there is a random behavior : some idle cpu > will use original capacity whereas others will use the capped value > set by non idle CPUs. You must have consistent behavior across all > idle cpus. > > Then, if it's not a problem why adding the if (!idle_cpu(cpu))
To capture the signal value from a running CPU, which then I pass into the em_cpu_energy() in path 2/2. My apologies for confusion, this can be just local variable for patch 1/2.
I can create the _cpu_cap as local variable inside this loop, just for this patch. Then in patch 2/2 I will remove it and define above the loop, to be available for the call to em_cpu_energy().
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