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

Beata Michalska
2021-05-26T12:16:02Z

On Wed, May 26, 2021 at 11:52:25AM +0200, Dietmar Eggemann wrote:
> On 25/05/2021 12:29, Beata Michalska wrote:
> > On Tue, May 25, 2021 at 10:53:07AM +0100, Valentin Schneider wrote:
> >> On 24/05/21 23:55, Beata Michalska wrote:
> >>> On Mon, May 24, 2021 at 07:01:04PM +0100, Valentin Schneider wrote:
> >>>> On 24/05/21 11:16, Beata Michalska wrote:
>
> [...]
>
> >>>>> +static inline int
> >>>>> +asym_cpu_capacity_classify(struct sched_domain *sd,
> >>>>> + const struct cpumask *cpu_map)
> >>>>> +{
> >>>>> + int sd_asym_flags = SD_ASYM_CPUCAPACITY | SD_ASYM_CPUCAPACITY_FULL;
> >>>>> + struct asym_cap_data *entry;
> >>>>> + int asym_cap_count = 0;
> >>>>> +
> >>>>> + if (list_is_singular(&asym_cap_list))
> >>>>> + goto leave;
> >>>>> +
> >>>>> + list_for_each_entry(entry, &asym_cap_list, link) {
> >>>>> + if (cpumask_intersects(sched_domain_span(sd), entry->cpu_mask)) {
> >>>>> + ++asym_cap_count;
> >>>>> + } else {
> >>>>> + /*
> >>>>> + * CPUs with given capacity might be offline
> >>>>> + * so make sure this is not the case
> >>>>> + */
> >>>>> + if (cpumask_intersects(entry->cpu_mask, cpu_map)) {
> >>>>> + sd_asym_flags &= ~SD_ASYM_CPUCAPACITY_FULL;
> >>>>> + if (asym_cap_count > 1)
> >>>>> + break;
> >>>>> + }
> >>>>
> >>>> Readability nit: That could be made into an else if ().
> >>> It could but then this way the -comment- gets more exposed.
> >>> But that might be my personal perception so I can change that.
> >>
> >> As always those are quite subjective! Methink something like this would
> >> still draw attention to the offline case:
> >>
> >> /*
> >> * Count how many unique capacities this domain covers. If a
> >> * capacity isn't covered, we need to check if any CPU with
> >> * that capacity is actually online, otherwise it can be
> >> * ignored.
> >> */
> >> if (cpumask_intersects(sched_domain_span(sd), entry->cpu_mask)) {
> >> ++asym_cap_count;
> >> } else if (cpumask_intersects(entry->cpu_mask, cpu_map)) {
> >> sd_asym_flags &= ~SD_ASYM_CPUCAPACITY_FULL;
> >> if (asym_cap_count > 1)
> >> break;
> >> }
> > Noted.
> > Will wait for some more comments before sending out 'polished' version.
>
> 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).
>
> Something like this (separating asym_cap_list iteration and flags
> construction would be easier for me. But like already said here,
> it's subjective.
> I left the two optimizations (list_is_singular(), break on asym_cap_count
> > 1) out for now. asym_cap_list shouldn't have > 4 entries (;-)).
>
> static inline int
> asym_cpu_capacity_classify(struct sched_domain *sd,
> const struct cpumask *cpu_map)
> {
> int sd_span_match = 0, cpu_map_match = 0, flags = 0;
> struct asym_cap_data *entry;
>
> list_for_each_entry(entry, &asym_cap_list, link) {
> if (cpumask_intersects(sched_domain_span(sd), entry->cpu_mask))
> ++sd_span_match;
> else if (cpumask_intersects(cpu_map, entry->cpu_mask))
> ++cpu_map_match;
> }
>
> WARN_ON_ONCE(!sd_span_match);
>
> if (sd_span_match > 1) {
> flags |= SD_ASYM_CPUCAPACITY;
> if (!cpu_map_match)
> flags |= SD_ASYM_CPUCAPACITY_FULL;
> }
>
> return flags;
> }
So I planned to drop the list_is_singular check as it is needless really.
Otherwise, I am not really convinced by the suggestion. I could add comments
around current version to make it more ..... 'digestible' but I'd rather
stay with it as it seems more compact to me (subjective).

>
> BTW, how would this mechanism behave on a system with SMT and asymmetric CPU
> capacity? Something EAS wouldn't allow but I guess asym_cap_list will be
> constructed and the SD_ASYM_CPUCAPACITY_XXX flags will be set?
Yes, the list would get created and flags set. I do not think there is
a difference with current approach (?). So EAS would be disabled (it only cares
about SD_ASYM_CPUCAPACITY_FULL flag) but the misift might still kick in.

---
BR
B.