linux-kernel - Re: [PATCH v2 1/3] sched: introduce distinct per-cpu load average

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Date:	Mon, 22 Oct 2012 13:10:40 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrea Righi <andrea@...terlinux.com>
Cc:	Paul Menage <paul@...lmenage.org>, Ingo Molnar <mingo@...hat.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 1/3] sched: introduce distinct per-cpu load average

On Sat, 2012-10-20 at 21:06 +0200, Andrea Righi wrote:
> @@ -383,13 +383,7 @@ struct rq {
>         struct list_head leaf_rt_rq_list;
>  #endif
>  

> +       unsigned long __percpu *nr_uninterruptible; 

This is O(nr_cpus^2) memory..


> +unsigned long nr_uninterruptible_cpu(int cpu)
> +{
> +       struct rq *this = cpu_rq(cpu);
> +       unsigned long val = 0;
> +       int i;
> +
> +       for_each_online_cpu(i)
> +               val += per_cpu(*this->nr_uninterruptible, i);
> +
> +       return val;
> +}
> 
> 
I suspect you've got an accounting leak here on hot-plug.
> 
>  unsigned long nr_uninterruptible(void)
>  {
>         unsigned long i, sum = 0;
>  
>         for_each_possible_cpu(i)
> -               sum += cpu_rq(i)->nr_uninterruptible;
> +               sum += nr_uninterruptible_cpu(i);
>  
>         /*
>          * Since we read the counters lockless, it might be slightly

And this makes O(n^2) runtime!


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