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

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Message-ID: <jhjeesyw96u.mognet@arm.com>
Date:   Wed, 08 Apr 2020 11:42:14 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....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 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.


>  extern void init_dl_bw(struct dl_bw *dl_b);