Re: [PATCH v5 2/3] sched/topology: Rework CPU capacity asymmetry detection

Beata Michalska
2021-05-27T12:32:34Z

On Thu, May 27, 2021 at 02:22:52PM +0200, Dietmar Eggemann wrote:
> On 27/05/2021 09:03, Peter Zijlstra wrote:
> > On Wed, May 26, 2021 at 11:52:25AM +0200, Dietmar Eggemann wrote:
> >
> >> For me asym_cpu_capacity_classify() is pretty hard to digest ;-) But I
> >> wasn't able to break it. It also performs correctly on (non-existing SMT)
> >> layer (with sd span eq. single CPU).
> >
> > This is the simplest form I could come up with this morning:
> >
> > static inline int
> > asym_cpu_capacity_classify(struct sched_domain *sd,
> > const struct cpumask *cpu_map)
> > {
> > struct asym_cap_data *entry;
> > int i = 0, n = 0;
> >
> > list_for_each_entry(entry, &asym_cap_list, link) {
> > if (cpumask_intersects(sched_domain_span(sd), entry->cpu_mask))
> > i++;
> > else
> > n++;
> > }
> >
> > if (WARN_ON_ONCE(!i) || i == 1) /* no asymmetry */
> > return 0;
> >
> > if (n) /* partial asymmetry */
> > return SD_ASYM_CPUCAPACITY;
> >
> > /* full asymmetry */
> > return SD_ASYM_CPUCAPACITY | SD_ASYM_CPUCAPACITY_FULL;
> > }
> >
> >
> > The early termination and everything was cute; but this isn't
> > performance critical code and clarity is paramount.
>
> This is definitely easier to grasp.
>
> What about the missing `if (cpumask_intersects(entry->cpu_mask,
> cpu_map))` condition in the else path to increment n?
>
> Example:
>
> cpus = {[446 446] [871 871] [1024 1024]}
>
> So 3 asym_cap_list entries.
>
> After hp'ing out CPU4 and CPU5:
>
> DIE: 'partial asymmetry'
>
> In case we would increment n only when the condition is met, we would
> have `full asymmetry`.
>
> I guess we want to allow EAS task placement, hence have
> sd_asym_cpucapacity set in case there are only 446 and 871 left?
>
I will rewrite the function as per all the suggestions and make things ....
more readable.

---
BR
B.