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

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Message-ID: <20200414114032.wigdlnegism6qqns@e107158-lin.cambridge.arm.com>
Date:   Tue, 14 Apr 2020 12:40:32 +0100
From:   Qais Yousef <qais.yousef@....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>,
        Juri Lelli <juri.lelli@...hat.com>,
        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>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] sched/deadline: Improve admission control for
 asymmetric CPU capacities

On 04/09/20 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
> 
> DEFINE_PER_CPU(cpumask_var_t, dl_bw_mask);
> 
> dl_bw_cpus(int i) {
> 
>     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;
> 
>     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.

I haven't followed this discussion closely, so I could be missing something
here.

In sched_cpu_dying() we call set_rq_offline() which clears the cpu in
rq->rd->online.

So the way I read the code

	rd->online = cpumask_and(rd->span, cpu_active_mask)

But I could have easily missed some detail.

Regardless, it seems to me that DL is working around something not right in the
definition of rd->span or using the wrong variable.

My 2p :-). I have to go back and read the discussion in more detail.

Thanks

--
Qais Yousef