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

[Lukasz Luba]

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@xxxxxxxxxxx> 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@xxxxxxx>
> > > ---
> > >   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