Re: [RFC PATCH 3/3] sched/fair: Traverse cpufreq policies to detect capacity inversion

[Qais Yousef <qyousef@layalina.io>]

On 11/30/22 19:27, Rafael J. Wysocki wrote:

> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 7c0dd57e562a..4bbbca85134b 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8856,23 +8856,20 @@ static void update_cpu_capacity(struct sched_domain *sd, int cpu)
> >          *   * Thermal pressure will impact all cpus in this perf domain
> >          *     equally.
> >          */
> > -       if (sched_energy_enabled()) {
> > +       if (static_branch_unlikely(&sched_asym_cpucapacity)) {
> >                 unsigned long inv_cap = capacity_orig - thermal_load_avg(rq);
> > -               struct perf_domain *pd = rcu_dereference(rq->rd->pd);
> > +               struct cpufreq_policy *policy, __maybe_unused *policy_n;
> >
> >                 rq->cpu_capacity_inverted = 0;
> >
> > -               SCHED_WARN_ON(!rcu_read_lock_held());
> > -
> > -               for (; pd; pd = pd->next) {
> > -                       struct cpumask *pd_span = perf_domain_span(pd);
> > +               for_each_active_policy_safe(policy, policy_n) {
> 
> 1. Is the "safe" part sufficient for protection against concurrent
> deletion and freeing of list entries?  cpufreq driver removal can do
> that AFAICS.

The freeing part is not safe probably. I need to research this more. Do you
have issues against the exportation of this traversal in principle?

Switching them to be RCU protected could be the best safe option, anything
against that too? I might not end up needing that. I need to dig more.

> 2. For a casual reader of this code it may not be clear why cpufreq
> policies matter here.

I'm looking for a way to traverse the list of capacities of the system and
know their related CPUs.

AFAICT this information already exists in the performance domains and
cpufreq_policy. Performance domains are conditional to energy model and
schedutil. So trying to switch to cpufreq_policy.

Assuming this question wasn't a request to add a comment :-)


Thanks!

--
Qais Yousef