linux-kernel - Re: [PATCH v2 2/6] sched/deadline: Optimize dl_bw

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Date:   Thu, 30 Apr 2020 16:25:15 +0530
From:   Pavan Kondeti <pkondeti@...eaurora.org>
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>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Qais Yousef <qais.yousef@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/6] sched/deadline: Optimize dl_bw_cpus()

On Mon, Apr 27, 2020 at 10:37:05AM +0200, Dietmar Eggemann wrote:
> Return the weight of the rd (root domain) span in case it is a subset
> of the cpu_active_mask.
> 
> Continue to compute the number of CPUs over rd span and cpu_active_mask
> when in hotplug.
> 
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
> ---
>  kernel/sched/deadline.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 504d2f51b0d6..4ae22bfc37ae 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -54,10 +54,16 @@ static inline struct dl_bw *dl_bw_of(int i)
>  static inline int dl_bw_cpus(int i)
>  {
>  	struct root_domain *rd = cpu_rq(i)->rd;
> -	int cpus = 0;
> +	int cpus;
>  
>  	RCU_LOCKDEP_WARN(!rcu_read_lock_sched_held(),
>  			 "sched RCU must be held");
> +
> +	if (cpumask_subset(rd->span, cpu_active_mask))
> +		return cpumask_weight(rd->span);
> +

Looks good to me. This is a nice optimization.

> +	cpus = 0;
> +
>  	for_each_cpu_and(i, rd->span, cpu_active_mask)
>  		cpus++;
>  
Do you know why this check is in place? Is it only to cover
the case of cpuset_cpu_inactive()->dl_cpu_busy()?

Thanks,
Pavan
> -- 
> 2.17.1
> 

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