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

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Message-ID: <8f00a2d4-2443-9656-2d51-6c5798fda552@arm.com>
Date:   Tue, 25 May 2021 13:59:30 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Beata Michalska <beata.michalska@....com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, valentin.schneider@....com,
        corbet@....net, rdunlap@...radead.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 2/3] sched/topology: Rework CPU capacity asymmetry
 detection

On 25/05/2021 11:30, Beata Michalska wrote:
> On Tue, May 25, 2021 at 10:25:36AM +0200, Dietmar Eggemann wrote:
>> On 24/05/2021 12:16, Beata Michalska wrote:

[...]

>>> @@ -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
>>
> If I got right the discussion you have pointed to, it was about using
> cpumask_var_t which is not the case here. I do not mind moving the code
> to use the array but I am not sure if this changes much. Looking at the
> code changes to support that (to_cpumask namely) it was introduced for
> cases where cpumask_var_t was not appropriate, which again isn't the case
> here.

Yeah, it was more about using `flexible array members` or allocating the
cpumask separately.

Looks like you're using some kind of a mixed approach:

(1) struct asym_cap_data {
        ...
        struct cpumask *cpu_mask;

(2) entry = kzalloc(sizeof(*entry) + cpumask_size(), GFP_KERNEL);

(3) entry->cpu_mask = (struct cpumask *)((char *)entry +
                      sizeof(*entry));

(4) cpumask_intersects(foo, entry->cpu_mask)


E.g. struct em_perf_domain has

(1)  struct em_perf_domain {
         ...
         unsigned long cpus[];

(2) like yours

(3) is not needed.

(4) cpumask_copy(em_span_cpus(pd), foo)

    with #define em_span_cpus(em) (to_cpumask((em)->cpus))

IMHO, it's better to keep this approach aligned between the different
data structures.