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

Dietmar Eggemann
2021-05-26T09:52:35Z

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;
}

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?