linux-kernel - Re: [PATCH 2/4] sched/deadline: Improve admission control for asymmetric CPU capacities

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Date:   Fri, 17 Apr 2020 14:19:45 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        luca abeni <luca.abeni@...tannapisa.it>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        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>,
        Qais Yousef <qais.yousef@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] sched/deadline: Improve admission control for
 asymmetric CPU capacities

On 09/04/20 19:29, Dietmar Eggemann wrote:

[...]

> 
> Maybe we can do a hybrid. We have rd->span and rd->sum_cpu_capacity and
> with the help of an extra per-cpu cpumask we could just

Hummm, I like the idea, but

> DEFINE_PER_CPU(cpumask_var_t, dl_bw_mask);
> 
> dl_bw_cpus(int i) {

This works if calls are always local to the rd we are interested into
(argument 'i' isn't used). Are we always doing that?

>     struct cpumask *cpus = this_cpu_cpumask_var_ptr(dl_bw_mask);
>     ...
>     cpumask_and(cpus, rd->span, cpu_active_mask);
> 
>     return cpumask_weight(cpus);
> }
> 
> and
> 
> dl_bw_capacity(int i) {
> 
>     struct cpumask *cpus = this_cpu_cpumask_var_ptr(dl_bw_mask);
>     ...
>     cpumask_and(cpus, rd->span, cpu_active_mask);
>     if (cpumask_equal(cpus, rd->span))
>         return rd->sum_cpu_capacity;

What if capacities change between invocations (with the same span)?
Can that happen?

> 
>     for_each_cpu(i, cpus)
>         cap += capacity_orig_of(i);
> 
>     return cap;
> }
> 
> So only in cases in which rd->span and cpu_active_mask differ we would
> have to sum up again.

Thanks,

Juri