linux-kernel - Re: [PATCH 1/4] sched/topology: Store root domain CPU capacity sum

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Date:   Wed, 8 Apr 2020 18:30:54 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Wei Wang <wvw@...gle.com>, Quentin Perret <qperret@...gle.com>,
        Alessio Balsini <balsini@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Qais Yousef <qais.yousef@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] sched/topology: Store root domain CPU capacity sum

On 08.04.20 14:29, Vincent Guittot wrote:
> On Wed, 8 Apr 2020 at 11:50, Dietmar Eggemann <dietmar.eggemann@....com> wrote:

[...]

>>  /**
>> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
>> index 8344757bba6e..74b0c0fa4b1b 100644
>> --- a/kernel/sched/topology.c
>> +++ b/kernel/sched/topology.c
>> @@ -2052,12 +2052,17 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>>         /* Attach the domains */
>>         rcu_read_lock();
>>         for_each_cpu(i, cpu_map) {
>> +               unsigned long cap = arch_scale_cpu_capacity(i);
> 
> Why do you replace the use of rq->cpu_capacity_orig by
> arch_scale_cpu_capacity(i) ?
> There is nothing about this change in the commit message

True. And I can change this back.

It seems though that the solution is not sufficient because of the
'rd->span &nsub cpu_active_mask' issue discussed under patch 2/4.

But this remind me of another question I have.

Currently we use arch_scale_cpu_capacity() more often (16 times) than
capacity_orig_of()/rq->cpu_capacity_orig .

What's hindering us to remove rq->cpu_capacity_orig and the code around
it and solely rely on arch_scale_cpu_capacity()? I mean the arch
implementation should be fast.