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

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Message-ID: <20210526121546.GA13262@e120325.cambridge.arm.com>
Date:   Wed, 26 May 2021 13:15:49 +0100
From:   Beata Michalska <beata.michalska@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, corbet@....net, rdunlap@...radead.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 2/3] sched/topology: Rework CPU capacity asymmetry
 detection

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.