linux-kernel - Re: [PATCH 2/9] Make cpuacct fields per cpu variables

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Date:	Mon, 19 Sep 2011 18:10:26 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, xemul@...allels.com,
	paul@...lmenage.org, lizf@...fujitsu.com, daniel.lezcano@...e.fr,
	mingo@...e.hu, jbottomley@...allels.com
Subject: Re: [PATCH 2/9] Make cpuacct fields per cpu variables

On Wed, 2011-09-14 at 17:04 -0300, Glauber Costa wrote:
> @@ -9258,11 +9253,17 @@ static int cpuacct_stats_show(struct cgroup *cgrp, struct cftype *cft,
>                 struct cgroup_map_cb *cb)
>  {
>         struct cpuacct *ca = cgroup_ca(cgrp);
> -       int i;
> +       int i, cpu;
> +       u64 acc[CPUACCT_STAT_NSTATS] = { 0, };
> +
> +       for_each_present_cpu(cpu) {
> +               u64 *vec = per_cpu_ptr(ca->cpustat, cpu);
> +               for (i = 0; i < CPUACCT_STAT_NSTATS; i++)
> +                       acc[i] += vec[i];
> +       }
>  
>         for (i = 0; i < CPUACCT_STAT_NSTATS; i++) {
> -               s64 val = percpu_counter_read(&ca->cpustat[i]);
> -               val = cputime64_to_clock_t(val);
> +               s64 val = cputime64_to_clock_t(acc[i]);
>                 cb->fill(cb, cpuacct_stat_desc[i], val);
>         }
>         return 0; 

The changelog doesn't mention this function is a lot more expensive now.

Someone might have daft software polling this frequently.. at least its
still fully preemptible so its not much of a problem, but it is
something to be aware of.
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