Re: [PATCH v5 09/23] PM: EM: Use runtime modified EM for CPUs energy estimation in EAS

Lukasz Luba
2023-12-19T08:31:16Z

Hi Qais and Xuewen,

On 12/19/23 04:03, Xuewen Yan wrote:
> On Mon, Dec 18, 2023 at 1:59 AM Qais Yousef <qyousef@layalina.io> wrote:
>>
>> On 11/29/23 11:08, Lukasz Luba wrote:
>>> The new Energy Model (EM) supports runtime modification of the performance
>>> state table to better model the power used by the SoC. Use this new
>>> feature to improve energy estimation and therefore task placement in
>>> Energy Aware Scheduler (EAS).
>>
>> nit: you moved the code to use the new runtime em table instead of the one
>> parsed at boot.
>>
>>>
>>> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
>>> ---
>>> include/linux/energy_model.h | 16 ++++++++++++----
>>> 1 file changed, 12 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
>>> index 1e618e431cac..94a77a813724 100644
>>> --- a/include/linux/energy_model.h
>>> +++ b/include/linux/energy_model.h
>>> @@ -238,6 +238,7 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
>>> unsigned long max_util, unsigned long sum_util,
>>> unsigned long allowed_cpu_cap)
>>> {
>>> + struct em_perf_table *runtime_table;
>>> unsigned long freq, scale_cpu;
>>> struct em_perf_state *ps;
>>> int cpu, i;
>>> @@ -255,7 +256,14 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
>>> */
>>> cpu = cpumask_first(to_cpumask(pd->cpus));
>>> scale_cpu = arch_scale_cpu_capacity(cpu);
>>> - ps = &pd->table[pd->nr_perf_states - 1];
>>> +
>>> + /*
>>> + * No rcu_read_lock() since it's already called by task scheduler.
>>> + * The runtime_table is always there for CPUs, so we don't check.
>>> + */
>>
>> WARN_ON(rcu_read_lock_held()) instead?
>
> I agree, or SCHED_WARN_ON(!rcu_read_lock_held()) ?

I disagree here. This is a sched function in hot path and as comment
says:

-----------------------
* This function must be used only for CPU devices. There is no validation,
* i.e. if the EM is a CPU type and has cpumask allocated. It is called
from
* the scheduler code quite frequently and that is why there is not checks.
-----------------------

We don't have to put the checks or warnings everywhere in the kernel
functions. Especially hot one like this one.

As you might not notice, we don't even check if the pd->cpus is not NULL

Regards,
Lukasz