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Subject: Re: [PATCH v5 2/3] sched/topology: Rework CPU capacity asymmetry
 detection
To:     Beata Michalska <beata.michalska@arm.com>,
        linux-kernel@vger.kernel.org
Cc:     peterz@infradead.org, mingo@redhat.com, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, valentin.schneider@arm.com,
        corbet@lwn.net, rdunlap@infradead.org, linux-doc@vger.kernel.org
References: <20210524101617.8965-1-beata.michalska@arm.com>
 <20210524101617.8965-3-beata.michalska@arm.com>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Message-ID: <cdb4e3a4-569f-1dc2-be22-c0128250996a@arm.com>
Date:   Tue, 25 May 2021 10:25:36 +0200
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On 24/05/2021 12:16, Beata Michalska wrote:

[...]

> Rework the way the capacity asymmetry levels are being detected,
> allowing to point to the lowest topology level (for a given CPU), where
> full set of available CPU capacities is visible to all CPUs within given
> domain. As a result, the per-cpu sd_asym_cpucapacity might differ across
> the domains. This will have an impact on EAS wake-up placement in a way
> that it might see different rage of CPUs to be considered, depending on

s/rage/range ;-)

[...]

> @@ -1266,6 +1266,112 @@ static void init_sched_groups_capacity(int cpu, struct sched_domain *sd)
>  	update_group_capacity(sd, cpu);
>  }
>  
> +/**
> + * Asymmetric CPU capacity bits
> + */
> +struct asym_cap_data {
> +	struct list_head link;
> +	unsigned long    capacity;
> +	struct cpumask   *cpu_mask;

Not sure if this has been discussed already but shouldn't the flexible
array members` approach known from struct sched_group, struct
sched_domain or struct em_perf_domain be used here?
IIRC the last time this has been discussed in this thread:
https://lkml.kernel.org/r/20200910054203.525420-2-aubrey.li@intel.com

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 0de6eef91bc8..03e492e91bd7 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1271,8 +1271,8 @@ static void init_sched_groups_capacity(int cpu,
struct sched_domain *sd)
  */
 struct asym_cap_data {
        struct list_head link;
-       unsigned long    capacity;
-       struct cpumask   *cpu_mask;
+       unsigned long capacity;
+       unsigned long cpumask[];
 };

 /*
@@ -1299,14 +1299,14 @@ asym_cpu_capacity_classify(struct sched_domain *sd,
                goto leave;

        list_for_each_entry(entry, &asym_cap_list, link) {
-               if (cpumask_intersects(sched_domain_span(sd),
entry->cpu_mask)) {
+               if (cpumask_intersects(sched_domain_span(sd),
to_cpumask(entry->cpumask))) {
                        ++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)) {
+                       if
(cpumask_intersects(to_cpumask(entry->cpumask), cpu_map)) {
                                sd_asym_flags &= ~SD_ASYM_CPUCAPACITY_FULL;
                                if (asym_cap_count > 1)
                                        break;
@@ -1332,7 +1332,6 @@ asym_cpu_capacity_get_data(unsigned long capacity)
        if (WARN_ONCE(!entry, "Failed to allocate memory for asymmetry
data\n"))
                goto done;
        entry->capacity = capacity;
-       entry->cpu_mask = (struct cpumask *)((char *)entry +
sizeof(*entry));
        list_add(&entry->link, &asym_cap_list);
 done:
        return entry;
@@ -1349,7 +1348,7 @@ static void asym_cpu_capacity_scan(void)
        int cpu;

        list_for_each_entry(entry, &asym_cap_list, link)
-               cpumask_clear(entry->cpu_mask);
+               cpumask_clear(to_cpumask(entry->cpumask));

        entry = list_first_entry_or_null(&asym_cap_list,
                                         struct asym_cap_data, link);
@@ -1361,11 +1360,11 @@ static void asym_cpu_capacity_scan(void)
                if (!entry || capacity != entry->capacity)
                        entry = asym_cpu_capacity_get_data(capacity);
                if (entry)
-                       __cpumask_set_cpu(cpu, entry->cpu_mask);
+                       __cpumask_set_cpu(cpu, to_cpumask(entry->cpumask));
        }

        list_for_each_entry_safe(entry, next, &asym_cap_list, link) {
-               if (cpumask_empty(entry->cpu_mask)) {
+               if (cpumask_empty(to_cpumask(entry->cpumask))) {
                        list_del(&entry->link);
                        kfree(entry);
                }

[...]