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

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Message-ID: <d2ace353-cdf3-c22a-2b19-7fa33281fe27@arm.com>
Date:   Thu, 9 Apr 2020 15:50:10 +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 19:03, Vincent Guittot wrote:
> On Wed, 8 Apr 2020 at 18:31, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>
>> 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.
>>ap
>> 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.
> 
> Or we can do the opposite and only use capacity_orig_of()/rq->cpu_capacity_orig.
> 
> Is there a case where the max cpu capacity changes over time ? So I
> would prefer to use cpu_capacity_orig which is a field of scheduler
> instead of always calling an external arch specific function

I see. So far it only changes during startup.

And it looks like that asym_cpu_capacity_level() [topology.c] would fail
if we would use capacity_orig_of() instead of arch_scale_cpu_capacity().

post_init_entity_util_avg() [fair.c] and sugov_get_util()
[cpufreq_schedutil.c] would be temporarily off until
update_cpu_capacity() has updated cpu_rq(cpu)->cpu_capacity_orig.

compute_energy() [fair.c] is guarded by sched_energy_enabled() from
being used at startup.

scale_rt_capacity() could be changed in case we call it after the
cpu_rq(cpu)->cpu_capacity_orig = arch_scale_cpu_capacity(cpu) in
update_cpu_capacity().

The Energy Model (and CPUfreq cooling) code would need
capacity_orig_of() exported. arch_scale_cpu_capacity() currently is
exported via include/linux/sched/topology.h.

I guess Pelt and 'scale invariant Deadline bandwidth enforcement' should
continue using arch_scale_cpu_capacity() in sync with
arch_scale_freq_capacity().

IMHO it's hard to give clear advice when to use the one or the other.

We probably don't want to set cpu_rq(cpu)->cpu_capacity_orig in the arch
cpu scale setter. We have arch_scale_cpu_capacity() to decouple that.