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 09:42:51 +0100 |
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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 idle CPU, which all signals in that function has 0 [1].
This will cause that we don't check what is a new value stored by thermal cpufreq_cooling for the thermal pressure [2]. We should feed that value into the 'signal' machinery inside the __update_blocked_others() [3]. Unfortunately, in a corner case there's a flag (rq->has_blocked_load) which blocks the check of a raw thermal value and prevents feeding it into thermal pressure signal (since it's a long idle CPU, there is no load) [4].
It has implication on this patch, because I cannot e.g. take first CPU from the PD mask and blindly check it's thermal pressure, because it can be idle for a long time. I don't want to have two loop, first just for taking the latest thermal pressure for the PD. Thus, I want to re-use the existing loop to take the latest information from non-idle CPU and pass use.
Regards, Lukasz
[1] https://elixir.bootlin.com/linux/latest/source/kernel/sched/fair.c#L7909 [2] https://elixir.bootlin.com/linux/latest/source/drivers/thermal/cpufreq_cooling.c#L494 [3] https://elixir.bootlin.com/linux/latest/source/kernel/sched/fair.c#L7958 [4] https://elixir.bootlin.com/linux/latest/source/kernel/sched/fair.c#L8433
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