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

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Date:   Wed, 8 Apr 2020 14:26:05 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     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>,
        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>,
        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 08.04.20 12:42, Valentin Schneider wrote:
> 
> On 08/04/20 10:50, Dietmar Eggemann wrote:
>> +++ b/kernel/sched/sched.h
>> @@ -304,11 +304,14 @@ void __dl_add(struct dl_bw *dl_b, u64 tsk_bw, int cpus)
>>       __dl_update(dl_b, -((s32)tsk_bw / cpus));
>>  }
>>
>> +static inline unsigned long rd_capacity(int cpu);
>> +
>>  static inline
>> -bool __dl_overflow(struct dl_bw *dl_b, int cpus, u64 old_bw, u64 new_bw)
>> +bool __dl_overflow(struct dl_bw *dl_b, int cpu, u64 old_bw, u64 new_bw)
>>  {
>>       return dl_b->bw != -1 &&
>> -	       dl_b->bw * cpus < dl_b->total_bw - old_bw + new_bw;
>> +	       cap_scale(dl_b->bw, rd_capacity(cpu)) <
>> +	       dl_b->total_bw - old_bw + new_bw;
>>  }
>>
> 
> I don't think this is strictly equivalent to what we have now for the SMP
> case. 'cpus' used to come from dl_bw_cpus(), which is an ugly way of
> writing
> 
>      cpumask_weight(rd->span AND cpu_active_mask);
> 
> The rd->cpu_capacity_orig field you added gets set once per domain rebuild,
> so it also happens in sched_cpu_(de)activate() but is separate from
> touching cpu_active_mask. AFAICT this mean we can observe a CPU as !active
> but still see its capacity_orig accounted in a root_domain.

I see what you mean.

The

int dl_bw_cpus(int i) {
    ...
    for_each_cpu_and(i, rd->span, cpu_active_mask)
        cpus++;
    ...
}

should be there to handle the 'rd->span &nsub cpu_active_mask' case.

We could use a similar implementation for s/cpus/capacity:

unsigned long dl_bw_capacity(int i) {
    ...
    for_each_cpu_and(i, rd->span, cpu_active_mask)
        cap += arch_scale_cpu_capacity(i);
    ...
}

[...]