Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH 1/4] sched/topology: Store root domain CPU capacity sum
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Luca Abeni <luca.abeni@santannapisa.it>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Wei Wang <wvw@google.com>, Quentin Perret <qperret@google.com>,
        Alessio Balsini <balsini@google.com>,
        Pavan Kondeti <pkondeti@codeaurora.org>,
        Patrick Bellasi <patrick.bellasi@matbug.net>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Qais Yousef <qais.yousef@arm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>
References: <20200408095012.3819-1-dietmar.eggemann@arm.com>
 <20200408095012.3819-2-dietmar.eggemann@arm.com>
 <CAKfTPtC4_+dTddLdoFMdzUvsXwWyi3bUOXcg9kstC8RzZS_a+A@mail.gmail.com>
 <42cc3878-4c57-96ba-3ebd-1b4d4ef87fae@arm.com>
 <CAKfTPtDS_qwPH+TwoFWPz6QRzG1N=t46ZvGN=w6zbOtnGAdOeQ@mail.gmail.com>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Message-ID: <d2ace353-cdf3-c22a-2b19-7fa33281fe27@arm.com>
Date:   Thu, 9 Apr 2020 15:50:10 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.4.1
MIME-Version: 1.0
In-Reply-To: <CAKfTPtDS_qwPH+TwoFWPz6QRzG1N=t46ZvGN=w6zbOtnGAdOeQ@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 08.04.20 19:03, Vincent Guittot wrote:
> On Wed, 8 Apr 2020 at 18:31, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>>
>> On 08.04.20 14:29, Vincent Guittot wrote:
>>> On Wed, 8 Apr 2020 at 11:50, Dietmar Eggemann <dietmar.eggemann@arm.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.