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

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Date:   Tue, 14 Apr 2020 17:27:21 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Quentin Perret <qperret@...gle.com>,
        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>,
        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 14.04.20 14:45, Quentin Perret wrote:
> On Wednesday 08 Apr 2020 at 19:03:57 (+0200), Vincent Guittot wrote:
>> 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
> 
> Note however that using arch_scale_cpu_capacity() would be more
> efficient, especially on non-arm/arm64 systems where it is a
> compile-time constant.

or essentially all ARCHs not defining it.

> 
> It's probably a matter of personal taste, but I find rq->cpu_capacity_orig
> superfluous. It wastes space without actually giving you anything no?
> Anybody remembers why it was introduced in the first place?

v3.18 arm providing arch_scale_cpu_capacity()
v4.1  introduction of rq->cpu_capacity_orig
v4.10 arm64 providing arch_scale_cpu_capacity()
...

So it's down to the question of 'minimizing the scheduler calls to an
external arch function' vs 'efficiency'.